wxPython error "can't set focus to invalid window"

[omission9]

An application I have written is, suddenly it seems, throwing the
following error after being left in use for several hours. This does
not seem to occur with lighter usage. I am not 100% sure what causes
this? Anyone have any advice? Seen it before?

Traceback (most recent call last):
  File "PythonCardPrototype\binding.pyo", line 304, in _dispatch
  File "wxPython\events.pyo", line 23, in GetEventType
wxPython.wxc.wxPyAssertionError: C++ assertion "wxAssertFailure"
failed in e:\Projects\wx2.4\src\msw\window.cpp(454): can't set focus
to invalid window