topStack returns empty?

[Richard Gaskin]

The topstack function should return the topmost window of the lowest 
mode, but under what circumstances (if any) should it return empty?

I would have thought that should never occur, since the engine requires 
at least one stack open at all times.  In my circumstance it should be 
even simpler, as there is one toplevel stack open and several palette 
windows -- yet under circumstances I have yet to pin down a recipe for, 
I have been able to query the topstack in the middle of a complex 
routine and it returns empty.

Why?

-- 
  Richard Gaskin
  Fourth World Media Corporation
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