Program: 70540
Project: 70540
Buggy source:
@Override public void onChildAdded(DataSnapshot dataSnapshot,String s){
  final Chat chat=new Chat((String)dataSnapshot.getValue());
  db.getReference().child("String_Node_Str").child(chat.getChat_id()).child("String_Node_Str").limitToLast(1).addValueEventListener(new ValueEventListener(){
    @Override public void onDataChange(    DataSnapshot dataSnapshot){
      for (      DataSnapshot snap : dataSnapshot.getChildren()) {
        db.getReference().child("String_Node_Str").child((String)snap.getValue()).addListenerForSingleValueEvent(new ValueEventListener(){
          @Override public void onDataChange(          DataSnapshot dataSnapshot){
            chat.setLast_message((String)dataSnapshot.child("String_Node_Str").getValue());
            chat.setTime((String)dataSnapshot.child("String_Node_Str").getValue());
            db.getReference().child("String_Node_Str").child(chat.getChat_id()).child("String_Node_Str").addListenerForSingleValueEvent(new ValueEventListener(){
              @Override public void onDataChange(              DataSnapshot dataSnapshot){
                for (                DataSnapshot snap : dataSnapshot.getChildren())                 if (!auth.getCurrentUser().getUid().equals((String)snap.getValue())) {
                  db.getReference().child("String_Node_Str").child((String)snap.getValue()).child("String_Node_Str").addListenerForSingleValueEvent(new ValueEventListener(){
                    @Override public void onDataChange(                    DataSnapshot dataSnapshot){
                      chat.setChat_name((String)dataSnapshot.getValue());
                      adapter.update(chat);
                    }
                    @Override public void onCancelled(                    DatabaseError databaseError){
                    }
                  }
);
                  break;
                }
              }
              @Override public void onCancelled(              DatabaseError databaseError){
              }
            }
);
          }
          @Override public void onCancelled(          DatabaseError databaseError){
          }
        }
);
        break;
      }
    }
    @Override public void onCancelled(    DatabaseError databaseError){
    }
  }
);
}

Fixed version (ground truth):
@Override public void onChildAdded(DataSnapshot dataSnapshot,String s){
  final Chat chat=new Chat((String)dataSnapshot.getValue());
  db.getReference().child("String_Node_Str").child(chat.getChat_id()).child("String_Node_Str").addListenerForSingleValueEvent(new ValueEventListener(){
    @Override public void onDataChange(    DataSnapshot dataSnapshot){
      chat.setPhoto_url((String)dataSnapshot.getValue());
      db.getReference().child("String_Node_Str").child(chat.getChat_id()).child("String_Node_Str").limitToLast(1).addValueEventListener(new ValueEventListener(){
        @Override public void onDataChange(        DataSnapshot dataSnapshot){
          for (          DataSnapshot snap : dataSnapshot.getChildren()) {
            db.getReference().child("String_Node_Str").child((String)snap.getValue()).addListenerForSingleValueEvent(new ValueEventListener(){
              @Override public void onDataChange(              DataSnapshot dataSnapshot){
                chat.setLast_message((String)dataSnapshot.child("String_Node_Str").getValue());
                chat.setTime((String)dataSnapshot.child("String_Node_Str").getValue());
                db.getReference().child("String_Node_Str").child(chat.getChat_id()).child("String_Node_Str").addListenerForSingleValueEvent(new ValueEventListener(){
                  @Override public void onDataChange(                  DataSnapshot dataSnapshot){
                    for (                    DataSnapshot snap : dataSnapshot.getChildren())                     if (!auth.getCurrentUser().getUid().equals((String)snap.getValue())) {
                      db.getReference().child("String_Node_Str").child((String)snap.getValue()).child("String_Node_Str").addListenerForSingleValueEvent(new ValueEventListener(){
                        @Override public void onDataChange(                        DataSnapshot dataSnapshot){
                          chat.setChat_name((String)dataSnapshot.getValue());
                          adapter.update(chat);
                        }
                        @Override public void onCancelled(                        DatabaseError databaseError){
                        }
                      }
);
                      break;
                    }
                  }
                  @Override public void onCancelled(                  DatabaseError databaseError){
                  }
                }
);
              }
              @Override public void onCancelled(              DatabaseError databaseError){
              }
            }
);
            break;
          }
        }
        @Override public void onCancelled(        DatabaseError databaseError){
        }
      }
);
    }
    @Override public void onCancelled(    DatabaseError databaseError){
    }
  }
);
}