This crash can be reproduced by adding a graph editor to a details customization. This crashes because the graph editor is holding a strong reference to the graph object on the actor component during the level switch. While changing levels the actor is deselected and the details panel's objects are set to null, but the associated treeview which is visualizing the details view doesn't actually refresh until the next tick preventing the level from unloading since the widget is causing the actor to be referenced.
1. Load the attached sample project, and TestMap1.
2. Select "Actor" in TestMap1.
3. Select the "Graph" component in the details panel.
4. Load map 2.
Result: Crash because map 1 can't be unloaded.
Expected Result: Level loads correctly.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-54352 in the post.