AssertionError cursor_position is None

I use xonsh as my shell with prompt-toolkit 3.0.4. Occasionally, I'll be typing in the prompt and get this traceback:

Unhandled exception in event loop:
  File "/Library/Frameworks/Python.framework/Versions/3.8/lib/python3.8/asyncio/events.py", line 81, in _run
    self._context.run(self._callback, *self._args)
  File "/Users/jaraco/.local/pipx/venvs/xonsh/lib/python3.8/site-packages/prompt_toolkit/eventloop/utils.py", line 72, in schedule
    func()
  File "/Users/jaraco/.local/pipx/venvs/xonsh/lib/python3.8/site-packages/prompt_toolkit/application/application.py", line 456, in redraw
    self._redraw()
  File "/Users/jaraco/.local/pipx/venvs/xonsh/lib/python3.8/site-packages/prompt_toolkit/application/application.py", line 523, in _redraw
    self.context.run(run_in_context)
  File "/Users/jaraco/.local/pipx/venvs/xonsh/lib/python3.8/site-packages/prompt_toolkit/application/application.py", line 509, in run_in_context
    self.renderer.render(self, self.layout)
  File "/Users/jaraco/.local/pipx/venvs/xonsh/lib/python3.8/site-packages/prompt_toolkit/renderer.py", line 568, in render
    layout.container.preferred_height(size.columns, size.rows).preferred,
  File "/Users/jaraco/.local/pipx/venvs/xonsh/lib/python3.8/site-packages/prompt_toolkit/layout/containers.py", line 324, in preferred_height
    dimensions = [
  File "/Users/jaraco/.local/pipx/venvs/xonsh/lib/python3.8/site-packages/prompt_toolkit/layout/containers.py", line 325, in <listcomp>
    c.preferred_height(width, max_available_height) for c in self._all_children
  File "/Users/jaraco/.local/pipx/venvs/xonsh/lib/python3.8/site-packages/prompt_toolkit/layout/containers.py", line 792, in preferred_height
    return self.content.preferred_height(width, max_available_height)
  File "/Users/jaraco/.local/pipx/venvs/xonsh/lib/python3.8/site-packages/prompt_toolkit/layout/containers.py", line 324, in preferred_height
    dimensions = [
  File "/Users/jaraco/.local/pipx/venvs/xonsh/lib/python3.8/site-packages/prompt_toolkit/layout/containers.py", line 325, in <listcomp>
    c.preferred_height(width, max_available_height) for c in self._all_children
  File "/Users/jaraco/.local/pipx/venvs/xonsh/lib/python3.8/site-packages/prompt_toolkit/layout/containers.py", line 2595, in preferred_height
    return self.content.preferred_height(width, max_available_height)
  File "/Users/jaraco/.local/pipx/venvs/xonsh/lib/python3.8/site-packages/prompt_toolkit/layout/containers.py", line 1620, in preferred_height
    return self._merge_dimensions(
  File "/Users/jaraco/.local/pipx/venvs/xonsh/lib/python3.8/site-packages/prompt_toolkit/layout/containers.py", line 1648, in _merge_dimensions
    preferred = get_preferred()
  File "/Users/jaraco/.local/pipx/venvs/xonsh/lib/python3.8/site-packages/prompt_toolkit/layout/containers.py", line 1613, in preferred_content_height
    return self.content.preferred_height(
  File "/Users/jaraco/.local/pipx/venvs/xonsh/lib/python3.8/site-packages/prompt_toolkit/layout/controls.py", line 639, in preferred_height
    content = self.create_content(width, height=1)  # Pass a dummy '1' as height.
  File "/Users/jaraco/.local/pipx/venvs/xonsh/lib/python3.8/site-packages/prompt_toolkit/layout/controls.py", line 761, in create_content
    document = buffer.document
  File "/Users/jaraco/.local/pipx/venvs/xonsh/lib/python3.8/site-packages/prompt_toolkit/buffer.py", line 524, in document
    return self._document_cache[
  File "/Users/jaraco/.local/pipx/venvs/xonsh/lib/python3.8/site-packages/prompt_toolkit/cache.py", line 98, in __missing__
    result = self.get_value(*key)
  File "/Users/jaraco/.local/pipx/venvs/xonsh/lib/python3.8/site-packages/prompt_toolkit/document.py", line 98, in __init__
    assert cursor_position is None or cursor_position <= len(text), AssertionError(

Exception cursor_position=9, len_text=4
Press ENTER to continue...                                                                                                                                                
~ $ cd ~/p/che

Sometimes, I hit enter and the error repeats. Other times, I hit Enter and can continue.

I've just updated to prompt_toolkit==3.0.5. I'll report back here if the issue repeats.

Start with the traceback path through prompt_toolkit/buffer.py, cache.py, document.py, and layout/controls.py, focusing on how cursor_position=9 and len_text=4 arise during redraw. Reproduce the intermittent prompt failure with the reported xonsh and prompt-toolkit versions, then add a regression test and verify that repeated redraws no longer raise the assertion.