Assertion failed: Child->IsChildOf(Parent) [File:D:/Build/++UE4/Sync/Engine/Source/Runtime/CoreUObject/Private/UObject/UObjectGlobals.cpp] [Line: 3169] NewObject called with invalid class, XXXX must be a child of PanelSlot.
Error appears after dragging a UWidget onto widget XXXX of type UContentWidget (a panel).
Widget XXXX inherits from UContentWidget. Previously it inherited from UNamedSlot. Before changing the parent class I had removed all of the widgets of XXXX from the UAssets. I have also cleared all caches. Could it be that the engine saved this old connection between the widget class and its old parent class somewhere else? How can I remove that data to fix my blueprints? Right now my editor constantly crashes with this error.