[lmi] Assertion 'GetEventHandler == this' failed

[Greg Chicares]

To reproduce:
  File | New | Census
  Census | Edit cell
  change "Withdrawal" so that it holds an empty string
  OK
  Census | Add cell
Now--in the census manager, without opening the tabbed dialog--click
"Withdrawal" in each row until the ellipsis button shows. (You don't
have to click the button--just make sure it shows, in both rows.)
Then
  File | Exit | No

I see a messagebox with the following contents (retyped manually
because 'wine' doesn't seem to let me copy and paste them):

Assertion 'GetEventHandler == this' failed (any pushed event handlers must have 
been removed).
[file ../src/common/wincmn.cpp, line 476]

Vadim--Should this be fixed in lmi, or in wx? We're about to update
wx, but if this calls for a wx change, we'll wait.