[lmi] Anomaly: empty messagebox

[Greg Chicares]

To reproduce:
  File | New | Illustration
Clear any input-sequence textcontrol (so that no text remains),
then click its ellipsis button. I see a messagebox with the
textcontrol's name in its titlebar, which contains "OK" and
"Cancel" buttons, but no message.

I looked in 'input_sequence_entry.cpp', but I can't see how it
could produce this messagebox. Vadim, can you figure this out?
Could it be that this is InputSequenceEntry::DoOpenEditor()
showing nothing at all? IOW, is this the intended behavior of
the following code path?

    if(intervals.empty())
        {
        // have single row (initial state)
        add_row();
        return;
        }

If that's the explanation, then maybe the right thing to do
is to establish !intervals.empty() as a ctor postcondition;
I was thinking of doing that anyway.