void OnBeforeSequenceChange()
        {
            treeView          = null;
            m_MarkerHeaderGUI = null;
            m_TimeAreaDirty   = true;

            state.Reset();
            m_PlayableLookup.ClearPlayableLookup();

            // clear old editors to caches, like audio previews, get flushed
            CustomTimelineEditorCache.ClearCache <ClipEditor>();
            CustomTimelineEditorCache.ClearCache <MarkerEditor>();
            CustomTimelineEditorCache.ClearCache <TrackEditor>();

            m_PreviousMasterSequence = state.masterSequence.asset;
        }