title: Items in RealtimeDatabase not added to collection
body: <pre><code>HashSet&lt;String&gt; timeSet = new HashSet&lt;&gt;();
  
  MainActivity.dbreference.addValueEventListener(new ValueEventListener() {
  @Override
  public void onDataChange(@NonNull @NotNull DataSnapshot snapshot) {
  for (DataSnapshot s : snapshot.child(&quot;users&quot;).child(MainActivity
                                .currentUser.getUid()).child(&quot;timetable&quot;).getChildren()) 
      {
          timeSet.add(s.getKey());
      }
  }
  
  @Override
  public void onCancelled(@NonNull @NotNull DatabaseError error) {}});
  
  Log.d(TAG, timeSet.toString());
  </code></pre>
  <p>This code was supposed to added to timeSet all keys, which are in this path (they are there), but output to log was showed, what set is empty</p>
  <p><strong>Why?</strong></p>