code: void setUndoView(@NonNull final View undoView) {
          if (mUndoView != null) {
              removeView(mUndoView);
          }
          mUndoView = undoView;
          mUndoView.setVisibility(GONE);
          addView(mUndoView);
      }
docs: Sets the undo {@link android.view.View}. Removes any existing primary {@code View} if present, and sets the visibility of the {@code undoView} to {@link #GONE}.