Sequencer: Undoing creation of a new section can call section drawing code with viewmodel tree in invalid state

The Section reference in ISequencerSection::GetSectionHeight can be in the process of garbage collection during an undo of a section addition- this causes things like GetTypedOuter<UMovieSceneTrack>() returning nullptr.

This seems to be because post undo, update code is called on the virtual geometry before the view model node tree is rebuilt.

See UDN at: https://udn.unrealengine.com/0D5QP00000QbSfN0AV?fromCase=1. 

And I can also reproduce the assert in the FCinematicShotSection::GetSectionHeight()

• clone the repo: https://github.com/EpicGames/UnrealEngine
• select 'release' branch
• Setup.bat
• GenerateProjectFiles.bat
• set configuration to 'DebugEditor'
• add the modification in FCinematicShotSection::GetSectionHeight()

1. GetSubSectionObject().GetSequenceID();
2. // and/or
3. UMovieSceneCinematicShotTrack* track = Section->GetTypedOuter<UMovieSceneCinematicShotTrack>();
4. check( track );

• build/start
• create a new c++ project
• set configuration to 'DebugEditor' in c++ project
• build/start the c++ project
• reproduce the steps in the first post:

1. create a level sequence
2. add a shot track
3. insert shot
4. select the new section
5. Ctrl+C
6. Ctrl+V
7. Ctrl+Z -> assert in GetSequenceID() or check()

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