title: dataSnapshot has the object but getValue() will return null
body: <p>So I'm struggling with this issue for about three hours now. <br/>
  I have this user class<br/></p>
  
  <pre><code>public class User {
  private String user_id;
  private String userName;
  private long phoneNumber;
  
  public User() {
  }
  
  public User(String user_id, String userName, long phoneNumber) {
      this.user_id = user_id;
      this.userName = userName;
      this.phoneNumber = phoneNumber;
  }
  </code></pre>
  
  <p>And I want to fetch some instances of this class from Firebase Database I am using this code to do so:
  <br/> database reference;</p>
  
  <pre><code>User currentUser;
  DatabaseReference database = FirebaseDatabase.getInstance().getReference(); 
  DatabaseReference usersRef = database.child("users");
  </code></pre>
  
  <p>query code:</p>
  
  <pre><code>Query query = usersRef.orderByChild("user_id").equalTo(user_id);
                  query.addListenerForSingleValueEvent (new ValueEventListener() {
                      @Override
                      public void onDataChange(DataSnapshot dataSnapshot) {
                           currentUser = dataSnapshot.getValue(User.class);
  }
  </code></pre>
  
  <p>Funny thing: this code actually worked once but I made some changes I guess, and now the getValue() returns null;
  <br/>
   this is my database in firebase
  <br/>
  <a href="https://i.stack.imgur.com/U2Qwh.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/U2Qwh.png" alt="enter image description here"></a><br/>
  and when I debug the app the dataSnapshot actually contains the object I am looking for
  <br/>
  <a href="https://i.stack.imgur.com/HR9L9.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/HR9L9.jpg" alt="enter image description here"></a>
  <br/>
  but the currentUser will be an object with all its fields null;<br/>
  <a href="https://i.stack.imgur.com/UUVEa.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/UUVEa.jpg" alt="enter image description here"></a>
  <br/>
  any ideas? I'm getting really frustrated already :(
  Thanks in advance!</p>