Level Blueprint's Parent Class can be set to another Level Blueprint's generated class

You can assign the Parent Class of Level Blueprint to be the generated class of another Level Blueprint, which gives you the following "Graph is linked to object(s) in external map" error if you try to save the referencing level:

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I don't think this is a workflow we intend to support, so it should not be possible to select other level blueprint generated classes as the parent class for this level blueprint.

1. Create a new project and a new map
2. Save the map as "PersistentLevel"
3. From the levels window, create a new sub level (the default setting of Streaming Method: Blueprint should be fine)
4. Save the level once
5. In the Sub Level's Level Blueprint, open the Class Settings, set the Parent class to "PersistentLevel_C" (the generated level blueprint class for the persistent level you just made.
6. Try to save the sub level map, observe Graph Linked External Object error

You are also able to set the persistent level's parent class to the generated class of the sub level, which will cause the parent class to fail to save for the same reason.