There seem to be two issues here. First, the if-statement at the
beginning of the function will return prematurely if
`self._private.widgets[index]` exists. There seems to be a
missing `not` there.
Second, index 1 is interpreted as the top of the stack (although the
documentation says otherwise), but the widget is inserted at the end of
`self._private.widgets`, so it gets pushed to the bottom
instead of the top.
13e41a2a7b