Check before and after media item reference

    private void removeAdjacentTransitions(MediaItem mediaItem) {

        final Transition beginTransition = mediaItem.getBeginTransition();

        if (beginTransition != null) {

            if (beginTransition.getAfterMediaItem() != null) {

                beginTransition.getAfterMediaItem().setEndTransition(null);

            }

            beginTransition.invalidate();

            mTransitions.remove(beginTransition);

        }

        final Transition endTransition = mediaItem.getEndTransition();

        if (endTransition != null) {

            if (endTransition.getBeforeMediaItem() != null) {

                endTransition.getBeforeMediaItem().setBeginTransition(null);

            }

            endTransition.invalidate();

            mTransitions.remove(endTransition);

        }