Unloading a World Partition region during nested Level Instance inline editing prevents exiting editing mode

In the case of a single-layer Level Instance, actors are still referenced even when the region is unloaded.
However, in the case of nested Level Instances, the level being edited is discarded when the region is unloaded.

1. Place an static mesh in the level
2. Create a Level Instance (LI_A) from the right-click context menu
3. Place another mesh at a distant location
4. Create another Level Instance (LI_B) from the right-click context menu, then create a nested Level Instance (LI_C)

   ParsistentLevel
     + LI_A
          + StaticMesh A
     + LI_C
          + LI_B
               + StaticMesh B
   

   (repro project is attached in this case)

5. Load each Level Instance from the World Partition tab
6. Start inline editing for LI_B
7. In the World Partition tab, unload the region that includes LI_B and LI_C
8. The Cancel button on the screen is grayed out
   1.  Pressing Save button crashes the editor
   2. Starting to edit another Level Instance also causes a crash