UITester does not interact well with GuiTestAssistant in Python 3.8

An issue was observed when using `UITester` with `GuiTestAssistant.assertEventuallyTrueInGui` in Python 3.8. The issue cannot be reproduced in python 3.6. For the test the following packages were used:

- traitsui 7.4.0
- pyface 7.4.2
- pyside6 6.3.2

There is a sample code that can reproduce the error (taken from traitsui/tests/editors/test_table_editor.py line 512):

```
class TestTableEditor(BaseTestMixin, GuiTestAssistant, unittest.TestCase):
def setUp(self):
BaseTestMixin.setUp(self)
GuiTestAssistant.setUp(self)

def tearDown(self):
BaseTestMixin.tearDown(self)
GuiTestAssistant.tearDown(self)

def test_table_editor_check_display_with_tester(self):
object_list = ObjectListWithSelection(
values=[ListItem(other_value=0)]
)
tester = UITester()
with tester.create_ui(object_list, dict(view=select_row_view)) as ui:
self.assertEventuallyTrueInGui(lambda: True)
wrapper = tester.find_by_name(ui, "values").locate(Cell(0, 1))

actual = wrapper.inspect(DisplayedText())
self.assertEqual(actual, "0")

object_list.values[0].other_value = 123

actual = wrapper.inspect(DisplayedText())
self.assertEqual(actual, "123")
```

Note that if you remove the line `self.assertEventuallyTrueInGui(lambda: True)` then the test works fine. When you leave it then you get an error on the following line: `ValueError: No editors can be found with name 'values'. Found these: []`