ActionBar complains about ActionSeparator already having a parent - kivy

Question

When using NavigationToolbar2Kivy from garden.matplotlib in a deeply nested widget (it's why I could not setup a simple working example showing the problem), I am thrown the following exception:
WidgetException: Cannot add <kivy.uix.actionbar.ActionSeparator object at 0x147956fa0>, it already has a parent <kivy.uix.actionbar.ActionOverflow object at 0x1478e44b0>
I could fix it by introducing the following changes in actionbar.py:
In ActionView._layout_random(), replace super_add(child, group_index) by:
if child.parent:
child.parent.remove_widget(child)
super_add(child, group_index)
over_add(child) by:
if child.parent:
child.parent.remove_widget(child)
over_add(child)
and super_add(child) by:
if child.parent:
child.parent.remove_widget(child)
super_add(child)
In ActionView._layout_all(), replace super_add(child) by:
if child.parent:
child.parent.remove_widget(child)
super_add(child)
I think that the problem comes from ActionView.remove_widget() and the complicated interactions between _list_action_item, _list_action_group and hidden_items, that I could not sort out.
As these changes are most probably harmless, I suggest these changes are included to improve ActionBar robustness.

Answer 1

The ActionOverflow widget is not supposed to contain any ActionSeparator, as shown by its overloaded add_widget method.
But in ActionView._layout_random() (in the very last lines), items in hidden_items are added to overflow_group (which is an ActionOverflow widget) using the inherited add_widget method, not the overloaded one.
So, in ActionView._layout_random(), it is necessary to prevent overflow_group to receive any ActionSeparator, so it is necessary to replace:
over_add(child)
by:
if not isinstance(child, ActionSeparator):
over_add(child)
This identifies and fixes the problem.