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;
}