Hi Emad,
Since the ListBox Format Manager just stores the different Format Strings, it might be difficult to identify. At least, you are lucky as your problem was that of more columns. I have had GPF's in previous Clarion versions. The only check that can be done is to compare the Original Format String with the Default on Initialisation and offer to reset all if they are different. But this would also cause problems as a simple change of a Picture Clause could trigger this.
By the way, Mark's suggestions was right but the only difference is that the PROP:Format assignment is done in the class. Check the ListFormatManagerClass in \Clarion6\libsrc\BRWEXT.clw.
Regards