guiDesigner crash logs are saved to a log.txt file, located in the same installation folder as guiDesigner itself.
By default, this is
C:\Program Files (x86)\CommandFusion\log.txt
You can also open the log.txt file directly from within guiDesigner by going to
File > Edit > Preferences… > View Log from the main menu.
More info on iViewer crash logs can be found here