Hello Elringus,
If you are removing all widgets from the viewport to prevent them from showing up in the next level this is no longer needed in 4.8. All widgets are removed when changing levels by default (The custom cursor will NOT be removed). If there is another reason for removing the widgets from the viewport before changing levels I have another solution for you. The Example is below. I hope that this information helps. This worked for me on my end.
Get all Widgets of Class (UserWidget) > For Each Loop > Array element == Custom Cursor Widget > If equal to > Do nothing> Else > Remove from viewport > Loop Complete > Open Level
Example in Blueprints:
Make it a great day