Crash in UWidgetAnimation::CreateDirectorInstance when switching resolution

Hello,

We've encountered a crash in UWidgetAnimation::CreateDirectorInstance when repeatedly switching resolutions. We've had it for a while but it's not until recently that we've found a consistent repro.

Our understanding is that it's a lifetime issue. Sequence of events:

1. Viewport resizing triggers Slate to rebuild its widgets
2. Some widget with an active animation gets destroyed/marked for delete
3. UMGSequenceTickManager::ForceFlush is called
4. During Flush MovieSceneEventSystem::TriggerEvents fires
5. Consequently CreateDirectorInstance is called on UWidgetAnimation
6. GetPlaybackContex() returns null
7. CastChecked LogFatal (crash)

Steps to Reproduce

The game UI must contain a UUserWidget with an active widget animation that has an event track (blueprint event keys firing during playback)

1. Run game in Editor Standalone Game mode
2. Access custom built-in video options menu
3. Switch to fullscreen exclusive window mode
4. Change resolutions until crash

CastLogError D:\MyProject-workspace\Engine\Source\Runtime\CoreUObject\Private\Templates\Casts.cpp (10)
UWidgetAnimation::CreateDirectorInstance D:\MyProject-workspace\Engine\Source\Runtime\UMG\Private\Animation\WidgetAnimation.cpp (341)
UE::MovieScene::FSequenceDirectorPlaybackCapability::GetOrCreateDirectorInstance D:\MyProject-workspace\Engine\Source\Runtime\MovieScene\Private\Evaluation\SequenceDirectorPlaybackCapability.cpp (83)
UMovieSceneEventSystem::TriggerEvents D:\MyProject-workspace\Engine\Source\Runtime\MovieSceneTracks\Private\Systems\MovieSceneEventSystems.cpp (173)
UMovieSceneEventSystem::TriggerAllEvents D:\MyProject-workspace\Engine\Source\Runtime\MovieSceneTracks\Private\Systems\MovieSceneEventSystems.cpp (130)
TBaseUObjectMethodDelegateInstance<...>::ExecuteIfSafe D:\MyProject-workspace\Engine\Source\Runtime\Core\Public\Delegates\DelegateInstancesImpl.h (713)
TMulticastDelegate<...>::Broadcast D:\MyProject-workspace\Engine\Source\Runtime\Core\Public\Delegates\DelegateSignatureImpl.inl (1076)
FMovieSceneEntitySystemRunner::GameThread_EventTriggerPhase D:\MyProject-workspace\Engine\Source\Runtime\MovieScene\Private\EntitySystem\MovieSceneEntitySystemRunner.cpp (1291)
FMovieSceneEntitySystemRunner::FlushNext D:\MyProject-workspace\Engine\Source\Runtime\MovieScene\Private\EntitySystem\MovieSceneEntitySystemRunner.cpp (457)
FMovieSceneEntitySystemRunner::FlushOutstanding D:\MyProject-workspace\Engine\Source\Runtime\MovieScene\Private\EntitySystem\MovieSceneEntitySystemRunner.cpp (550)
UUMGSequenceTickManager::ForceFlush D:\MyProject-workspace\Engine\Source\Runtime\UMG\Private\Animation\UMGSequenceTickManager.cpp (303)
UUMGSequenceTickManager::TickWidgetAnimations D:\MyProject-workspace\Engine\Source\Runtime\UMG\Private\Animation\UMGSequenceTickManager.cpp (196)
TBaseUObjectMethodDelegateInstance<...>::ExecuteIfSafe D:\MyProject-workspace\Engine\Source\Runtime\Core\Public\Delegates\DelegateInstancesImpl.h (713)
TMulticastDelegate<...>::Broadcast D:\MyProject-workspace\Engine\Source\Runtime\Core\Public\Delegates\DelegateSignatureImpl.inl (1076)
FSlateApplication::TickAndDrawWidgets D:\MyProject-workspace\Engine\Source\Runtime\Slate\Private\Framework\Application\SlateApplication.cpp (1736)
FSlateApplication::Tick D:\MyProject-workspace\Engine\Source\Runtime\Slate\Private\Framework\Application\SlateApplication.cpp (1640)
FEngineLoop::Tick D:\MyProject-workspace\Engine\Source\Runtime\Launch\Private\LaunchEngineLoop.cpp (6016)
GuardedMain D:\MyProject-workspace\Engine\Source\Runtime\Launch\Private\Launch.cpp (190)
GuardedMainWrapper D:\MyProject-workspace\Engine\Source\Runtime\Launch\Private\Windows\LaunchWindows.cpp (123)
LaunchWindowsStartup D:\MyProject-workspace\Engine\Source\Runtime\Launch\Private\Windows\LaunchWindows.cpp (277)
WinMain D:\MyProject-workspace\Engine\Source\Runtime\Launch\Private\Windows\LaunchWindows.cpp (334)