Widget element (VerticalBox) return none when trying to add/clear children in graph.

In my project, vertical box variable always “Accessed none“ when is used as target in nodes “Add Children”, ‘Clear Children”.

Apparently ctrl-enter does not work here. So First - Created MasterWidget in character blueprint. Second - By event Master widget toggle visibility and add array of data, they go in loop and add as children to vertical box in MasterWidget. And VerticalBox (or ANY other element) return none as target in add/clear children nodes. What is goin on? How I can add widget as children to other widget element? Is there a check box somewhere I don’t see?