Renaming or Moving a Map with a Landscape in it results in an error and fails to save map

When renaming or moving a map with a landscape in it this will fail and an error prompt will appear.

1. Create a blank Blueprint project in 4.20
2. Add a Landscape to the level and then save it
3. Try to either rename the level or move the level into a different folder

Result: An error prompt appears: Can't save ../../../../../UE4/DeleteME/Content/NewMap2.umap: Graph is linked to private object(s) in an external package. External Object(s): LandscapeMaterialInstanceConstant_2 Try to find the chain of references to that object (may take some time)?

Expected: The level will be renamed or moved successfully.

Moving a map file with a landscape material causes issues