Getting error over the calculation on retrieved data from Fire base

I'm going to calculate the average rating of books.

Here,Rating and Counter is initialized in Firebase. I am getting fatal error on this line of my code sum = Double.parseDouble(r); which is

java.lang.NullPointerException: Attempt to invoke virtual method 'double java.lang.Double.doubleValue()' on a null object reference

But I could not find out the reason. It accurately retrieves both the counter value and rating without the rating calculation portion.

Here's the code and the database snapshot.

 public class GetReview { private FirebaseUser user; private Firebase mrootref; private String path2="hhhh"; private String c; private String r; private Double sum; // private String rate = " "; private Double avg; private Double counter; private Double rate; private double rounded; GetReview() { user = FirebaseAuth.getInstance().getCurrentUser(); Log.d("path1", path2); path2= myCategory.getPath(); Log.d("paths", path2); }; public void setReview(String bookname, Double rating) { rate= rating; //sum = sum+rate; Log.d("sum?", rating.toString()); path2 += "/"+bookname+"/"; mrootref = new Firebase(path2); mrootref.addValueEventListener(new com.firebase.client.ValueEventListener() { @Override public void onDataChange(com.firebase.client.DataSnapshot dataSnapshot) { for (com.firebase.client.DataSnapshot mshot : dataSnapshot.getChildren()) { Log.d("emmaList ", mshot.toString()); //Map<String, String> mmap = mshot.getValue(Map.class); // c = mshot.child("Counter:").getValue().toString(); //String review = mshot.getKey().toString(); //String emma = mshot.getKey().toString(); //Log.d(review, "I am the key"); if (mshot.getKey().toString().equals("Counter")) { c =String.valueOf( mshot.getValue()); Log.d(c, "I am counter"); } if (mshot.getKey().toString().equals("Rating")) { r =String.valueOf( mshot.getValue()); Log.d(r, "I am rate"); } //r = mmap.get("Rating"); counter = Double.parseDouble(c); counter++; sum = Double.parseDouble(r); sum = sum + rate; avg = sum / counter; sum = avg; Log.d(counter.toString(), "i am review"); //Log.d( avg.toString(),"i am avg"); BigDecimal bd = new BigDecimal(avg); bd = bd.round(new MathContext(3)); rounded = bd.doubleValue(); //Map revBook = new HashMap(); mrootref.child("Counter").setValue(counter.toString()); mrootref.child("Rating").setValue(Double.toString(rounded)); } } @Override public void onCancelled(FirebaseError firebaseError) { //Toast.makeText(GetReview.this, "Error in data retrieval", Toast.LENGTH_SHORT).show(); } }); } }