Why is UE4 crashin´ every time I delete anything?

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Assertion failed: [File:D:/Build/++UE4/Sync/Engine/Source/Runtime/CoreUObject/Private/UObject/Obj.cpp] [Line: 202] Renaming an object (Package /Game/ThirdPersonBP/Maps/ThirdPersonExampleMap_BuiltData) on top of an existing object (Package /Game/ThirdPersonExampleMap_2019-12-03-13-33-44_BuiltData) is not allowed

UE4Editor_Core
UE4Editor_Core
UE4Editor_Core
UE4Editor_Core
UE4Editor_CoreUObject
UE4Editor_Engine
UE4Editor_Engine
UE4Editor_CoreUObject
UE4Editor_CoreUObject
UE4Editor_CoreUObject
UE4Editor_CoreUObject
UE4Editor_Engine
UE4Editor_CoreUObject
UE4Editor_CoreUObject
UE4Editor_CoreUObject
UE4Editor_CoreUObject
UE4Editor_CoreUObject
UE4Editor_CoreUObject
UE4Editor_CoreUObject
UE4Editor_AssetTools
UE4Editor_AssetTools
UE4Editor_AssetTools
UE4Editor_UnrealEd
UE4Editor_UnrealEd
UE4Editor_UnrealEd
UE4Editor_UnrealEd
UE4Editor_UnrealEd
UE4Editor_LevelEditor
UE4Editor_LevelEditor
UE4Editor_Slate
UE4Editor_Slate
UE4Editor_UnrealEd
UE4Editor_LevelEditor
UE4Editor_Slate
UE4Editor_Slate
UE4Editor_Slate
UE4Editor_ApplicationCore
UE4Editor_ApplicationCore
UE4Editor_ApplicationCore
UE4Editor_ApplicationCore
user32
user32
UE4Editor_ApplicationCore
UE4Editor
UE4Editor
UE4Editor
UE4Editor
UE4Editor
kernel32
ntdll

Try doing a SaveAs… on the level, and edit that one.

If that fails, you have to make a new level, and copy everything from your old level to the new one. Not as bad as it sounds, just CTRL-A / copy / paste. But you will two copies of the editor open to do it…