title: Why am I getting the Stack Overflow error?
body: <p>I get a Stack Overflow error but I don't seem to know what's wrong. This is Database class helps me to get FirebaseDatabase information. </p>
  
  <pre><code> import 'package:firebase_database/firebase_database.dart';
      import 'User.dart';
      import 'dart:async';
  
  
      class FireDatabase{
        FireDatabase();
  
        DatabaseReference userRef = FirebaseDatabase.instance.reference().child("users");
        DatabaseReference transactionsRef = FirebaseDatabase.instance.reference().child("transactions");
        User user = new User();
  
  
       getDatabaseUser(String cUid){
          userRef.orderByChild(cUid).once().then((DataSnapshot data){
            if (data.value!=null) {
              return data.value;
            } else {
              print("prints - database.dart : data.value of user is empty/null");
              return null;
            }
          },onError: (e){
            print("prints - database.dart " + e.toString());
            return null;
          });
          return null;
        }
  
        getDatabaseTransactions(String cUid) {
          transactionsRef.orderByChild("transactions").once().then((DataSnapshot data){
            if(data.value!=null) {
              return data.value;
            } else {
              print("prints - database.dart : data.value of user is empty/null");
              return null;
            }
          },onError: (e){
            print("prints - database.dart " + e.toString());
            return null;
          });
          return null;
        }
  
        getDatabaseTransaction(String cUid) {
          transactionsRef.orderByChild(cUid).limitToFirst(1).once().then((DataSnapshot data){
            if(data.value!=null) {
              return data.value;
            } else {
              print("prints - database.dart : data.value of user is empty/null");
              return null;
            }
          },onError: (e){
            print("prints - database.dart " + e.toString());
            return null;
          });
          return null;
        }
  
      }
  </code></pre>