Unable to save a map file when editing a LevelInstance by placing multiple LevelInstances on a PersistentLevel

Can not save the map file, if trying to edit a LevelInstance in a PersistentLevel where multiple LevelInstances are placed. The workaround is to open and edit the LevelInstance as PersistentLevel. 

1. Open blank level.
2. Place 2 cube actors in the world.
3. Right crick in outliner and select [Level]->[Create LevelInstance...] to save level asset.
4. Place LevelInstance which created step3 in the world.
5. Select LevelInstance in outliner.
6. Select [Edit] button in details panel.
7. Click OpenBlueprint button in MenuBar and select [Sub-Levels]->[Edit (LevelInstance Name)]
8. Create PrintString node and connect to EventBeginPlay node.
9. Save LevelInstance asset.

Then, can not save a map file and output dialogue with following message.

The asset '/Game/Maps/NewMap' (NewMap.umap) failed to save.
Cancel: Stop saving all assets and return to the editor.
Retry: Attempt to save the asset again.
Continue: Skip saving this asset only. 

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