“Accessed None trying to read property CallFunc_Array_Get_Item”
It might be as simple as the *slot1 *not being a child of the Viewport. Uncheck *Top Level Only *and see if the error is still there.
If the error is thrown again, it means that the widget was never created in the first place or was destroyed after creation. This could happen when the owning parent gets Garbage Collected and no valid references to the pertinent widget remain anywhere else.