[lmi] wxmsw-2.9.0 regression: messagebox doubling

[Greg Chicares]

Run lmi with the argument that we never write in public, in order
to enable the "Test" menu. Then:

  alt-F N I Enter alt-T A F

or, if you please,

  File | New | Illustration
  OK
  Test | [...] application | Test fatal-error alert

Two fatal-error messageboxen appear. The desired behavior, which
is observed with wx-2.8.9, is that only one messagebox appears.

It doesn't matter whether the messageboxen are dismissed by
pressing Enter or clicking OK: there are still two of them.

The same behavior is observed with this change in commands:
- File | New | Illustration
- OK
+ File | New | Census
or with any of several other file types. What seems to matter is
whether at least one MDI-child window exists. It doesn't matter
whether the MDI child is minimized. Closing all MDI children
inhibits the double-messagebox behavior.