I have a Blueprint base-class that is nativized using the checkmark on Class Settings.
I then create a sub-class of that BP and do not think to check the value for the Nativize setting.
Subsequent “Launch” to (in my case Quest) result in sporadic crashes of the editor with an error containing the following output:
Assertion failed: !Export.ClassIndex.IsNull() [File:D:/Build/++UE4/Sync/Engine/Source/Runtime/CoreUObject/Private/UObject/SavePackage.cpp] [Line: 4957] Export SKEL_BP_Subclass_C /Game/Customer/BP/BP_Subclass.Default__SKEL_BP_Subclass_C class is not mapped when saving /Game/Customer/BP/BP_Subclass
Note The value for this checkbox is stored in DefaultEngine.ini; not on the Blueprint asset itself. (If you’re using version control make sure to submit Config.)
Update (2020-01-30) As of this morning this “fix” is no longer an effective cure for the problem. (Life comes at you fast.)