fix backtrace on opening program/resource on small screens

Fix regression introduced by f4696d8 ("add stretching to columns Name,
Type, Doc in variable panel").
This problem happens only on wxPython-3.0.

[TOPOLOGY]
NetName=None
Nodes=0x01
Node64Present=0x01
Node64Name=micromod
Node64DCFName=PEAK MicroMod.eds
EDSBaseName=eds