When packaging with nativization if you have a widget as a value in a TMap, the package will not finish building. The TMap will throw an error during the Package process that reads:
I believe that this is a bug as Subwidget is a child of UserWidget, which is a UClass.
Result: The Package does not build
Expected Result: The Package builds
Head over to the existingAnswerHub thread and let us know what's up.