title: method 'boolean java.lang.String.isEmpty()' on a null object reference
body: <p>After call Firebase RecyclerView, it shows me this error. After to call model.getAvatarUrl().isEmpty() because return null. Could you please help me, I stuck here. I called online and offline method and it recently shows me this error.</p>
  
  <p>FindFriendsActivity.java</p>
  
  <pre><code>adapter = new FirebaseRecyclerAdapter&lt;User, FindFriendsViewHolder&gt;(options) {
              @Override
              protected void onBindViewHolder(@NonNull final FindFriendsViewHolder viewHolder, final int position, @NonNull final User model)
              {
                  final String usersIDs = getRef(position).getKey();
  
                  UsersRef.child(usersIDs).addValueEventListener(new ValueEventListener() {
                      @Override
                      public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                          if (dataSnapshot.exists())
                          {
                              viewHolder.userName.setText(model.getName());
                              viewHolder.userStatus.setText(model.getStatus());
  
  
                              //Set Image
                              /*
                              if (model.getAvatarUrl().isEmpty()) {
                                  viewHolder.profileImage.setImageResource(R.drawable.profile_image);
                              } else
                              {
                                  Picasso.with(getBaseContext()).load(model.getAvatarUrl()).into(viewHolder.profileImage);
                              }*/
                              if (model.getAvatarUrl().isEmpty()){   //Error in this line
                                  viewHolder.profileImage.setImageResource(R.drawable.profile_image);
                              }
                              else {
                                  Picasso.with(getBaseContext()).load(model.getAvatarUrl()).into(viewHolder.profileImage);
                              }
  
                              viewHolder.itemView.setOnClickListener(new View.OnClickListener() {
                                  @Override
                                  public void onClick(View view)
                                  {
                                      String visit_user_id = getRef(position).getKey();
  
                                      Intent profileIntent = new Intent(FindFriendsActivity.this, ChatActivity.class);
                                      profileIntent.putExtra("visit_user_id", visit_user_id);
                                      profileIntent.putExtra("visit_user_name", model.getName());
                                      profileIntent.putExtra("visit_image", model.getAvatarUrl());
                                      startActivity(profileIntent);
                                  }
                              });
  
                          }
                      }
  
                      @Override
                      public void onCancelled(@NonNull DatabaseError databaseError) {
  
                      }
                  });
  
              }
  
              @NonNull
              @Override
              public FindFriendsViewHolder onCreateViewHolder(@NonNull ViewGroup viewGroup, int i)
              {
                  View itemView = LayoutInflater.from(viewGroup.getContext())
                          .inflate(R.layout.users_display_layout, viewGroup, false);
                  return new FindFriendsViewHolder(itemView);
              }
          };
  </code></pre>