Ensure when closing PIE during Level Sequence playback with spawnables in a streamed level

When a spawnable is spawned in a streamed level, exiting PIE will cause an ensure, because the world is not valid on the actor. 

1. Create a level with a blueprint streamed sublevel
2. Create a new level sequence in that level
3. Add a level visibility track to the sequence
4. Create a new visible visibility trigger in the track, or use an existing one, and add the sublelevel to the level names (right click > Properties > Level Names)
5.  Add a spawnable to the sequence by dragging an actor from the content browser directly to the sequence. 
6. Set the Spawnable Level to the same sublevel from steps 1 and 4
7. Save the sequence, and set the sequence actor to auto play
8. Save the level and PIE
9. Exit PIE before the playback ends by hitting esc

RESULT

An ensure will be hit (see callstack)

LogOutputDevice: Error: === Handled ensure: ===
LogOutputDevice: Error: Ensure condition failed: World [Link Removed] [Line: 242]
LogOutputDevice: Error: Stack:
LogOutputDevice: Error: [Callstack] 0x00007fff520e3c29 UE4Editor-LevelSequence.dll!<lambda_5e4a7c330d0dad93cac675e0035ef255>::operator()() [D:\_Release-4.26\Engine\Source\Runtime\LevelSequence\Private\LevelSequenceActorSpawner.cpp:242]
LogOutputDevice: Error: [Callstack] 0x00007fff520bf71a UE4Editor-LevelSequence.dll!FLevelSequenceActorSpawner::DestroySpawnedObject() [D:\_Release-4.26\Engine\Source\Runtime\LevelSequence\Private\LevelSequenceActorSpawner.cpp:242]
LogOutputDevice: Error: [Callstack] 0x00007fff520bf823 UE4Editor-LevelSequence.dll!FLevelSequenceSpawnRegister::DestroySpawnedObject() [D:\_Release-4.26\Engine\Source\Runtime\LevelSequence\Private\LevelSequenceSpawnRegister.cpp:41]
LogOutputDevice: Error: [Callstack] 0x00007fff514e818b UE4Editor-MovieScene.dll!FMovieSceneSpawnRegister::DestroyObjectsByPredicate() [D:\_Release-4.26\Engine\Source\Runtime\MovieScene\Private\MovieSceneSpawnRegister.cpp:89]
LogOutputDevice: Error: [Callstack] 0x00007fff514e27b7 UE4Editor-MovieScene.dll!FMovieSceneTrackEvaluator::CallSetupTearDown() [D:\_Release-4.26\Engine\Source\Runtime\MovieScene\Private\Evaluation\Instances\MovieSceneTrackEvaluator.cpp:409]
LogOutputDevice: Error: [Callstack] 0x00007fff51477192 UE4Editor-MovieScene.dll!UE::MovieScene::FSequenceInstance::Finish() [D:\_Release-4.26\Engine\Source\Runtime\MovieScene\Private\EntitySystem\MovieSceneSequenceInstance.cpp:210]
LogOutputDevice: Error: [Callstack] 0x00007fff5147736c UE4Editor-MovieScene.dll!FMovieSceneEntitySystemRunner::FinishInstance() [D:\_Release-4.26\Engine\Source\Runtime\MovieScene\Private\EntitySystem\MovieSceneEntitySystemRunner.cpp:498]
LogOutputDevice: Error: [Callstack] 0x00007fff514f6bcb UE4Editor-MovieScene.dll!FMovieSceneRootEvaluationTemplateInstance::Finish() [D:\_Release-4.26\Engine\Source\Runtime\MovieScene\Private\Evaluation\MovieSceneEvaluationTemplateInstance.cpp:124]
LogOutputDevice: Error: [Callstack] 0x00007fff5152a65c UE4Editor-MovieScene.dll!UMovieSceneSequencePlayer::StopInternal() [D:\_Release-4.26\Engine\Source\Runtime\MovieScene\Private\MovieSceneSequencePlayer.cpp:391]
LogOutputDevice: Error: [Callstack] 0x00007fff520c092f UE4Editor-LevelSequence.dll!ALevelSequenceActor::EndPlay() [D:\_Release-4.26\Engine\Source\Runtime\LevelSequence\Private\LevelSequenceActor.cpp:174]
LogOutputDevice: Error: [Callstack] 0x00007fff58878ae8 UE4Editor-Engine.dll!AActor::RouteEndPlay() [D:\_Release-4.26\Engine\Source\Runtime\Engine\Private\Actor.cpp:2261]
LogOutputDevice: Error: [Callstack] 0x00007fff57631681 UE4Editor-UnrealEd.dll!UEditorEngine::TeardownPlaySession() [D:\_Release-4.26\Engine\Source\Editor\UnrealEd\Private\PlayLevel.cpp:737]
LogOutputDevice: Error: [Callstack] 0x00007fff575f3653 UE4Editor-UnrealEd.dll!UEditorEngine::EndPlayMap() [D:\_Release-4.26\Engine\Source\Editor\UnrealEd\Private\PlayLevel.cpp:297]
LogOutputDevice: Error: [Callstack] 0x00007fff57082632 UE4Editor-UnrealEd.dll!UEditorEngine::Tick() [D:\_Release-4.26\Engine\Source\Editor\UnrealEd\Private\EditorEngine.cpp:1984]
LogOutputDevice: Error: [Callstack] 0x00007fff579b8dd6 UE4Editor-UnrealEd.dll!UUnrealEdEngine::Tick() [D:\_Release-4.26\Engine\Source\Editor\UnrealEd\Private\UnrealEdEngine.cpp:426]
LogOutputDevice: Error: [Callstack] 0x00007ff75bc39b6d UE4Editor-Win64-DebugGame.exe!FEngineLoop::Tick() [D:\_Release-4.26\Engine\Source\Runtime\Launch\Private\LaunchEngineLoop.cpp:4834]
LogOutputDevice: Error: [Callstack] 0x00007ff75bc5116c UE4Editor-Win64-DebugGame.exe!GuardedMain() [D:\_Release-4.26\Engine\Source\Runtime\Launch\Private\Launch.cpp:169]
LogOutputDevice: Error: [Callstack] 0x00007ff75bc5124a UE4Editor-Win64-DebugGame.exe!GuardedMainWrapper() [D:\_Release-4.26\Engine\Source\Runtime\Launch\Private\Windows\LaunchWindows.cpp:137]
LogOutputDevice: Error: [Callstack] 0x00007ff75bc6522d UE4Editor-Win64-DebugGame.exe!WinMain() [D:\_Release-4.26\Engine\Source\Runtime\Launch\Private\Windows\LaunchWindows.cpp:268]
LogOutputDevice: Error: [Callstack] 0x00007ff75bc67f5a UE4Editor-Win64-DebugGame.exe!__scrt_common_main_seh() [D:\agent\_work\9\s\src\vctools\crt\vcstartup\src\startup\exe_common.inl:288]
LogOutputDevice: Error: [Callstack] 0x00007ffff0db7034 KERNEL32.DLL!UnknownFunction []
LogOutputDevice: Error: [Callstack] 0x00007ffff271cec1 ntdll.dll!UnknownFunction []