Unable to register TclNotifier window class

[Gordon McMillan]

OK Tk wizards (hi, /F-bot!)

I'm getting "Unable to register TclNotifier window class".
What does this mean?

Details:
  Python 1.5.2b2 on NT SP4

I'm doing a tree control / form style window:
 a big frame
 another frame, packed 'left' with a Pmw.ScrolledListBox acting as a 
tree control.
 Then I have three different frames, each holding a "form" for 
displaying / editing a particular record type.
 When a new item in the tree control is selected, I "pack_forget" (if 
the record type has changed) the right hand frame, then "pack" the 
appropriate frame and fill the form.

 Works dandy for 2 of the three forms. Actually, the problem form 
fills properly, too, it just does a St Vitus dance until I kill the 
whole app (it appears to be continually re-packing itself). My 
console window has the mystery error.

 The problem form is the most complex (has the most widgets), but 
otherwise they are all clones of each other.

- Gordon