PopoverItem._configureRect() throws trying to access unmounted context when popover is open

[moescs]

Describe the bug
If you enable widget selection mode in VSCode's Widget Inspector while the popover is open, it triggers didChangeDependencies to schedule a _configureRect in PopoverItem. The problem is that the BuildContext becomes unmounted between didChangeDependencies and the post frame callback.

To Reproduce
Steps to reproduce the behavior:

1. Use VSCode.

2. Add a popover using showPopover().

3. Run the app using VSCode's debugger.

4. Open the Widget Inspector.

5. Open the popover in the app (trigger a call to showPopover()).

6. Click the toggle button to turn select widget mode on in the Widget Inspector.

7. BuildContextExtension.getWidgetLocalToGlobal() in _configureRect() throws the following error:

   FlutterError (Cannot get renderObject of inactive element.
   In order for an element to have a valid renderObject, it must be active, which means it is part of the tree.
   Instead, this element is in the _ElementLifecycle.defunct state.
   If you called this method from a State object, consider guarding it with State.mounted.
   The findRenderObject() method was called for the following element:
   StatelessElement#7e777(DEFUNCT))

Expected behavior
No error should be thrown.

Desktop (please complete the following information):

• OS: Windows 11
• Desktop app
• Version 10.0.22621 Build 22621

Additional comments
A possible solution is to add if (!widget.context.mounted) return; at the top of _configureRect(). I would generate a pull request, but I'm not sure if this would be exempt from testing when reading your guidelines. I can't think of a good way to test it.

[minikin]

@moescs Thank you for the report. I don't have access to a Windows machine.
Would you mind creating a PR with the fix and recording a short video to showcase that the issue is fixed?