Fix crash in QAccessible on macOS when parent is null

[kryksyh]

Qt's accessibility on macOS may crash if an accessible item with the ListItem role has no parent (e.g., after setAccessibleParent(nullptr), which triggers QAccessible::ParentChanged). Internals assume the parent is valid and dereference it (see qcocoaaccessibilityelement.mm:136)

Our implementation falls back to qApp->focusWindow() (AccessibilityController::parentIface()), so the parent is usually set, but when the app is inactive the crash could happen (e.g. during theme reloads or when inspected by Accessibility Inspector, or during the shutdown).

This PR reduces the chance of having no accessible parent during UI changes, minimizing these crashes.

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