Widget retains reference for widgets that have been replaced and deleted

Widget references remain even after the referenced widgets have been deleted. This happens if the widget was replaced with another user created widget.

1. Open a blank project
2. Create a new widget blueprint
3. Name it "ParentWidget"
4. Create a new widget blueprint
5. Name it "ChildWidget1"
6. Create a new widget blueprint
7. Name it "ChildWidget2"
8. Save and compile
9. Open "ParentWidget"
10. Add "ChildWidget1" to the canvas panel of "ParentWidget"
11. Select ChildWidget2 in the User created section (so that it will come up as a suggestion in the right click menu)
12. Right click "ChildWidget1" > Replace with > "ChildWidget2"
13. Delete ChildWidget2 (This should leave just the canvas panel)
14. Save and compile
15. Right click "ParentWidget" > Reference Viewer

Result: ParentWidget still shows a reference to ChildWidget1
Expected: The ChildWidget1 and ChildWidget2 would no longer be referenced after they have been deleted