[PCG] Create PCG Asset from Level crashes on loaded sub-level exports

PCG Level Asset export doesn't support exporting a level while it is loaded as a sub-level. We should warn the user instead of crashing.

1. Create a new empty level (Level_A)
2. Create a new empty level (Level_B)
3. Open Level_A
4. Open "Levels" window (Window -> Levels)
5. Add Level_B into Level_A (Levels drop down -> Add Existing -> Level_B)
6. Reopen Level_A to make sure changes are saved
7. Run the "Create PCG Assets from Level(s)" command on Level_B (right click on Level_B in the content browser -> Asset Actions -> Create PCG Assets from Level(s))