Bug report: Exception when closing a workbench with status bar

## Environment

OS: _Windows_
Python version: _3.10_
Toolkit: _Wx_

## Description

When creating a `Workbench` with a `status_bar`, the following exception is raised when closing the window:

```
Traceback (most recent call last):
File "D:\ANSYSDev\thirdparty_sources\pyface\pyface\dock\dock_sizer.py", line 4022, in CalcMin
dx, dy = self._contents.calc_min()
File "D:\ANSYSDev\thirdparty_sources\pyface\pyface\dock\dock_sizer.py", line 3190, in calc_min
dx, dy = item.calc_min(use_size)
File "D:\ANSYSDev\thirdparty_sources\pyface\pyface\dock\dock_sizer.py", line 2402, in calc_min
theme = self.theme
File "D:\ANSYSDev\thirdparty_sources\pyface\pyface\dock\dock_sizer.py", line 503, in _get_theme
return self.control.GetParent().owner.theme
RuntimeError: wrapped C/C++ object of type TraitsUIPanel has been deleted
```

## Steps to Reproduce

- Run the following script (adapted and minimized from the `workbench` example):

```python
from traits.etsconfig.api import ETSConfig
ETSConfig.toolkit = 'wx'

from traits.api import HasTraits, Int, Str
from pyface.api import GUI
from pyface.action.api import StatusBarManager
from pyface.workbench.api import WorkbenchWindow, Workbench


class ExampleWorkbenchWindow(WorkbenchWindow):
def _status_bar_manager_default(self):
return StatusBarManager(messages=['', ''])


class Person(HasTraits):
name = Str()
age = Int()

def __str__(self):
return self.name

if __name__ == "__main__":
gui = GUI()
fred = Person(name="fred", age=42)
workbench = Workbench(state_location=gui.state_location, window_factory=ExampleWorkbenchWindow)
window = workbench.create_window()
window.open()
window.edit(fred)

gui.start_event_loop()
```
- Close the application

## Other observations

The exception does _not_ occur if the window tab (`fred`) is closed before closing the Application.

Changing the toolkit to `'qt'` also makes the exception disappear.