Steps to Reproduce
Blank project.
* File -> New Level -> Empty Level. I named it “L_Instance”.
* Place an Actor in “L_Instance”.
* Content Browser -> Add -> Level Sequence. I named it “SE_Sequence”.
* Drag “SE_Sequence” into “L_Instance” (to create a LevelSequenceActor).
* Edit “SE_Sequence”.
* Add Actor to Sequencer: “Actor” (in “L_Instance”).
* Add a Transform track to Actor.
* Save “SE_Sequence” and “L_Instance”.
* File -> New Level -> Empty Level. I named it “L_World”.
* Drag “L_Instance” into “L_World” (to create a LevelInstance actor).
* Save “L_World”.
* Select “LI_Instance” in the Outliner. In Details, click “Edit”.
* Select “SE_Sequence” LevelSequence actor in Outliner. In Details, click “Open Level Sequence”.
* In the Level Editor, click “Exit” (5.4) / “Cancel” (5.5) to close the Level Instance Editor.
Observe crash.