code: @Override
      public void unregisterAdapterDataObserver(RecyclerView.AdapterDataObserver observer) {
          super.unregisterAdapterDataObserver(observer);
          if (mAdapter != null) {
              mAdapter.unregisterAdapterDataObserver(observer);
          }
      }
docs: overwrite the unregisterAdapterDataObserver to correctly forward all events to the FastAdapter
  
  @param observer