Insert failed. First exception on row 0; first error: REQUIRED_FIELD_MISSING, Required fields are missing: [Contact]: [Contact]

public with sharing class Apparel_PageController { public List<Trendsters_Order__c> orders{get;set;} public String contactId{get;set;} public Case objCase{get;set;} public Decimal totalAmount{get;set;} public Decimal cartSize{get;set;} public String currentProductId{get;set;} public List<Trendsterz_Inventory__c> inventries {get;set;} public List<Trendsters_Order__c> cart{get;set;} public Apparel_PageController(){ totalAmount = 0; cartSize = 0; cart = new List<Trendsters_Order__c>(); User objUser = [select ContactId from User where Id = :UserInfo.getUserId()]; contactId = objUser.ContactId; //contactId = '0037F00000a8HVg'; objCase = new Case(ContactId=contactId); inventries = [select Id,Brand__c, Designer_Name__c, Design_Name__c, Price__c,Image_Url__c from Trendsterz_Inventory__c LIMIT 10]; } public void fetchOrders(){ orders = [SELECT Id,Name,Brand__c,Contact__c,Contact__r.Name, Contact__r.Email, Designer_Name__c,Design_Name__c,Email__c,Price__c,Order_Status__c FROM Trendsters_Order__c WHERE Contact__c = :contactId]; } public void submitFeedback(){ insert objCase; } public void addToCart(){ for(Trendsterz_Inventory__c inventry : inventries){ if(currentProductId !=null && inventry.Id == currentProductId){ Trendsters_Order__c order = new Trendsters_Order__c(); system.debug('contactid -->'+contactId); order.Contact__c = contactId; order.Design_Name__c = inventry.Design_Name__c; order.Price__c= inventry.Price__c; cart.add(order); break; } } cartSize = cart.size(); // totalAmount = cart.size(); } public void payProduct(){ system.debug('cart -> ' + cart.size()); insert cart; } }