Android - Fragment not attached to activity

I have an activity which loads a fragment but when I press back button on the fragment it shows the error :-

"java.lang.IllegalStateException: Fragment ProductFragment{c46ba8a} not attached to Activity"

Code is below:-

Product Fragment:

public class ProductFragment extends Fragment implements TabLayout.OnTabSelectedListener, ViewPager.OnPageChangeListener { private static final String PRODUCT_DATA = "product_data"; private TabLayout tabLayout; private ViewPager viewPager; NestedScrollView nestedScrollView; //RecyclerView listView; //ImageView ivHeader; ProgressDialog pd; Bundle bundle; ViewPager viewPager1; private LinearLayout pager_indicator; public String DATA = "data"; public String PRODUCTS = "products"; //public String SPRODUCTS = "sproducts"; public String ID = "id"; public String NAME = "productName"; public String IMAGEURL = "productImg1"; public String DESCRIPTION = "description"; public String PRICE = "price"; public String DELIVERYTYPE = "deliverytype"; //String url = "https://chiraggohil.000webhostapp.com/product.php"; String url = "http://www.thinkdream.in/lunchbox2/product_api/getProductList"; String sliderImageUrl = "http://www.thinkdream.in/lunchbox2/assets/images/product/2.png"; ArrayList<Product> list = new ArrayList<>(); ArrayList<Product> sliderImagesList = new ArrayList<>(); private int dotsCount = 0; private ImageView[] dots; private static int CURRENT_PAGE = 0; ViewPagerSliderAdapter viewPagerSliderAdapter; ConnectivityManager cm; NetworkInfo activeNetwork; boolean isConnected; //private boolean load = false; @Override public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { final View v = inflater.inflate(R.layout.fragment_product, container, false); viewPager1 = (ViewPager) v.findViewById(R.id.viewpager); //listView = (RecyclerView) v.findViewById(R.id.gvProducts); nestedScrollView = (NestedScrollView) v.findViewById(R.id.nestedSV); pager_indicator = (LinearLayout) v.findViewById(R.id.viewPagerCountDots); //nestedScrollView.setFillViewport(true); //ivHeader = (ImageView) v.findViewById(R.id.ivHeader); bundle = new Bundle(); tabLayout = (TabLayout) v.findViewById(R.id.tabLayout); viewPager = (ViewPager) v.findViewById(R.id.pager); viewPagerSliderAdapter = new ViewPagerSliderAdapter(getActivity(), sliderImagesList); cm = (ConnectivityManager) getActivity().getSystemService(Context.CONNECTIVITY_SERVICE); activeNetwork = cm.getActiveNetworkInfo(); isConnected = activeNetwork != null && activeNetwork.isConnectedOrConnecting(); if (savedInstanceState != null) { sliderImagesList = savedInstanceState.getParcelableArrayList(PRODUCT_DATA); viewPager1.setAdapter(viewPagerSliderAdapter); viewPagerSliderAdapter.notifyDataSetChanged(); } else { if (isConnected) { new GetUrlData(getActivity()).execute(); } else { Toast.makeText(getActivity(), "You are not connected to the internet!", Toast.LENGTH_SHORT).show(); } } tabLayout.addTab(tabLayout.newTab().setText("Pizza")); tabLayout.addTab(tabLayout.newTab().setText("Sandwich")); tabLayout.addTab(tabLayout.newTab().setText("Burger")); Pager adapter = new Pager(getChildFragmentManager(), tabLayout.getTabCount()); viewPager.setAdapter(adapter); viewPager.addOnPageChangeListener(new TabLayout.TabLayoutOnPageChangeListener(tabLayout)); tabLayout.addOnTabSelectedListener(this); viewPager1.addOnPageChangeListener(this); return v; } @Override public void onSaveInstanceState(Bundle outState) { super.onSaveInstanceState(outState); outState.putParcelableArrayList(PRODUCT_DATA, sliderImagesList); } @Override public void onResume() { super.onResume(); pager_indicator.removeAllViews(); //Picasso.with(getActivity()).load(sliderImageUrl).into(ivHeader); } @Override public void onTabSelected(TabLayout.Tab tab) { viewPager.setCurrentItem(tab.getPosition()); } @Override public void onTabUnselected(TabLayout.Tab tab) { } @Override public void onTabReselected(TabLayout.Tab tab) { } @Override public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) { } @Override public void onPageSelected(int position) { for (int i = 0; i < dotsCount; i++) { dots[i].setImageDrawable(getResources().getDrawable(R.drawable.nonselecteditem_dot)); } if (position >= dotsCount) { dots[0].setImageDrawable(getResources().getDrawable(R.drawable.selecteditem_dot)); } else { dots[position].setImageDrawable(getResources().getDrawable(R.drawable.selecteditem_dot)); } } @Override public void onPageScrollStateChanged(int state) { } private class GetUrlData extends AsyncTask<Void, Void, Void> { GetUrlData(Context context) { pd = new MyCustomProgressDialog(context, R.style.NewDialog); } @Override protected void onPreExecute() { super.onPreExecute(); pd.setCancelable(false); pd.show(); } @Override protected Void doInBackground(Void... voids) { ServiceHandler sh = new ServiceHandler(); String result = sh.Getdata(url); //list.clear(); sliderImagesList.clear(); if (result != null) { try { list.clear(); JSONObject jsonObject = new JSONObject(result); JSONArray jsonArray = jsonObject.getJSONArray(DATA); //JSONArray jsonArray = jsonObject1.getJSONArray(PRODUCTS); for (int i = 0; i < jsonArray.length(); i++) { JSONObject jsonObject2 = jsonArray.getJSONObject(i); String id = jsonObject2.getString(ID); String name = jsonObject2.getString(NAME); String imgpath = jsonObject2.getString(IMAGEURL); String desc = jsonObject2.getString(DESCRIPTION); String price = jsonObject2.getString(PRICE); //String deliverytype = jsonObject2.getString(DELIVERYTYPE); Product product = new Product(); product.setId(id); product.setProductName(name); product.setProductImage(imgpath); product.setProductDescription(desc); product.setProductPrice(price); //list.add(product); sliderImagesList.add(product); } } catch (Exception e) { e.printStackTrace(); } } return null; } @Override protected void onPostExecute(Void aVoid) { pd.dismiss(); // Picasso.with(getActivity()).load(sliderImageUrl).into(ivHeader); if(isAdded()) { viewPagerSliderAdapter = new ViewPagerSliderAdapter(getActivity(), sliderImagesList); Log.e("slider",sliderImagesList.toString()); viewPager1.setAdapter(viewPagerSliderAdapter); viewPagerSliderAdapter.notifyDataSetChanged(); slider(); setUiPageViewController(); }else { Log.e("not added","not added"); } } } private void slider() { int NUM_PAGES = sliderImagesList.size(); final android.os.Handler handler = new android.os.Handler(); final Runnable runnable = new Runnable() { @Override public void run() { CURRENT_PAGE = viewPager1.getCurrentItem(); if (CURRENT_PAGE == 2) { CURRENT_PAGE = -1; } viewPager1.setCurrentItem(CURRENT_PAGE + 1, true); } }; Timer swipeTimer = new Timer(); swipeTimer.schedule(new TimerTask() { @Override public void run() { handler.post(runnable); } }, 100, 4000); } private void setUiPageViewController() { dotsCount = 3; dots = new ImageView[dotsCount]; for (int i = 0; i < dotsCount; i++) { dots[i] = new ImageView(getActivity()); dots[i].setImageDrawable(getResources().getDrawable(R.drawable.nonselecteditem_dot)); LinearLayout.LayoutParams params = new LinearLayout.LayoutParams( LinearLayout.LayoutParams.WRAP_CONTENT, LinearLayout.LayoutParams.WRAP_CONTENT ); params.setMargins(4, 0, 4, 0); pager_indicator.addView(dots[i], params); } dots[0].setImageDrawable(getResources().getDrawable(R.drawable.selecteditem_dot)); } } 

I don't know where I am wrong. I have searched for the error and even tried the solution from the link below:
Fragment MyFragment not attached to Activity
I ask questions on SO but I don't get response/answers. May be there is a small mistake, grammatical/formating problem, or a duplicate question but there is a possibility of my case that is different then others.
So please help and response.
Thanks.
--Edited--
HomeActivity :

public class HomeActivity extends AppCompatActivity implements View.OnClickListener { DrawerLayout drawerLayout; NavigationView navigationView; Toolbar toolbar; TextView tvActionTitle; SessionManager sessionManager; GPSTracker gps; int i = 0; private FrameLayout redCircle; private TextView countTextView; int cartcount = 0; Menu menu; LunchBoxDB lunchBoxDB; private String uid; SharedPreferences pref; private int totalquantity = 0; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_home); toolbar = (Toolbar) findViewById(R.id.toolbar); drawerLayout = (DrawerLayout) findViewById(R.id.drawer_layout); navigationView = (NavigationView) findViewById(R.id.navigation); tvActionTitle = (TextView) findViewById(R.id.tvActionTitle); Typeface custom_font = Typeface.createFromAsset(getAssets(), "fonts/Quicksand_Bold_Oblique.otf"); tvActionTitle.setTypeface(custom_font); setSupportActionBar(toolbar); //getSupportActionBar().setDisplayShowTitleEnabled(false); lunchBoxDB = new LunchBoxDB(this); pref = getSharedPreferences("loginPref", Context.MODE_PRIVATE); uid = pref.getString("id", null); ProductFragment pf = new ProductFragment(); getFragmentManager().beginTransaction().replace(R.id.content_frame, pf).commit(); sessionManager = new SessionManager(this); toolbar.setNavigationIcon(R.mipmap.ic_menu_white_24dp); toolbar.setNavigationOnClickListener(this); navigationView.setNavigationItemSelectedListener(new NavigationView.OnNavigationItemSelectedListener() { @Override public boolean onNavigationItemSelected(MenuItem item) { Fragment frag = null; int itemId = item.getItemId(); if (itemId == R.id.nav_home) { drawerLayout.closeDrawers(); Intent i = new Intent(HomeActivity.this, HomeActivity.class); startActivity(i); HomeActivity.this.finish(); } else if (itemId == R.id.nav_profile) { frag = new AccountFragment(); getFragmentManager().beginTransaction().replace(R.id.content_frame, frag).addToBackStack("Account").commit(); drawerLayout.closeDrawers(); } else if (itemId == R.id.nav_order) { drawerLayout.closeDrawers(); OrdersFragment of = new OrdersFragment(); getFragmentManager().beginTransaction().replace(R.id.content_frame, of).addToBackStack("Orders").commit(); } else if (itemId == R.id.nav_store_locator) { Intent i = new Intent(HomeActivity.this, LocationActivity.class); startActivity(i); drawerLayout.closeDrawers(); } else if (itemId == R.id.nav_logout) { drawerLayout.closeDrawers(); sessionManager.logoutUser(); } else if (itemId == R.id.nav_help) { drawerLayout.closeDrawers(); Intent i = new Intent(Intent.ACTION_DIAL); i.setData(Uri.parse("tel:+918460765785")); startActivity(i); } else if (itemId == R.id.nav_corpinquiry) { drawerLayout.closeDrawers(); CorpInquiryFragment crpf = new CorpInquiryFragment(); getFragmentManager().beginTransaction().replace(R.id.content_frame, crpf).addToBackStack("CorpInquiry").commit(); } return false; } }); } private void setNavigationDrawer() { if (drawerLayout.isDrawerOpen(GravityCompat.START)) { drawerLayout.closeDrawer(GravityCompat.START); } else { drawerLayout.openDrawer(GravityCompat.START); } } @Override public boolean onCreateOptionsMenu(Menu menu) { getMenuInflater().inflate(R.menu.nav_drawer, menu); final MenuItem alertMenuItem = menu.findItem(R.id.shopping_bag); FrameLayout rootView = (FrameLayout) alertMenuItem.getActionView(); redCircle = (FrameLayout) rootView.findViewById(R.id.view_alert_red_circle); countTextView = (TextView) rootView.findViewById(R.id.view_alert_count_textview); rootView.setOnClickListener(new View.OnClickListener() { @Override public void onClick(View v) { onOptionsItemSelected(alertMenuItem); } }); badgeupdate(); return super.onCreateOptionsMenu(menu); } void badgeupdate() { SQLiteDatabase db = lunchBoxDB.getReadableDatabase(); String query = "select sum(quantity) from cart where uid = " + uid; Cursor c = db.rawQuery(query, null); if (c != null && c.moveToFirst()) { totalquantity = c.getInt(0); } c.close(); cartcount = totalquantity; countTextView.setText(String.valueOf(cartcount)); redCircle.setVisibility((cartcount > 0) ? VISIBLE : GONE); } @Override public boolean onOptionsItemSelected(MenuItem item) { if (item.getItemId() == R.id.shopping_bag) { SharedPreferences pref = this.getSharedPreferences("couponPref", Context.MODE_PRIVATE); SharedPreferences.Editor editor = pref.edit(); editor.putString("discount", null); editor.apply(); CartFragment cf = new CartFragment(); getFragmentManager().beginTransaction().replace(R.id.content_frame, cf, "CartFragment").addToBackStack(null).commit(); } return super.onOptionsItemSelected(item); } @Override public void onBackPressed() { if (drawerLayout.isDrawerOpen(GravityCompat.START)) { drawerLayout.closeDrawer(GravityCompat.START); }/*else if(i==0){ Toast.makeText(this, "Press back again to exit!", Toast.LENGTH_SHORT).show(); i=1; }*/ else { //i=0; super.onBackPressed(); } } @Override protected void onResume() { super.onResume(); gps = new GPSTracker(this); } @Override public void onClick(View view) { setNavigationDrawer(); } } 

Logcat error :

FATAL EXCEPTION: main Process: com.pisac.foodrestaurant, PID: 29700 java.lang.IllegalStateException: Fragment ProductFragment{13525074} not attached to Activity at android.app.Fragment.getResources(Fragment.java:800) at com.pisac.foodrestaurant.ProductFragment.onPageSelected(ProductFragment.java:149) at android.support.v4.view.ViewPager.dispatchOnPageSelected(ViewPager.java:1967) at android.support.v4.view.ViewPager.scrollToItem(ViewPager.java:685) at android.support.v4.view.ViewPager.setCurrentItemInternal(ViewPager.java:669) at android.support.v4.view.ViewPager.setCurrentItemInternal(ViewPager.java:630) at android.support.v4.view.ViewPager.setCurrentItem(ViewPager.java:622) at com.pisac.foodrestaurant.ProductFragment$1.run(ProductFragment.java:244) at android.os.Handler.handleCallback(Handler.java:739) at android.os.Handler.dispatchMessage(Handler.java:95) at android.os.Looper.loop(Looper.java:135) at android.app.ActivityThread.main(ActivityThread.java:5910) at java.lang.reflect.Method.invoke(Native Method) at java.lang.reflect.Method.invoke(Method.java:372) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1405) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1200)