.pfg causes system crash

Hi Mike and Qspice team

I have found that .pfg the waveform viewer will crash if I change some nets name after saving .pfg

While it is reasonable for the viewer to crash if it cant find any waveform name that it suppose to print, however it is quite tricky as we are not normally review the .pfg. Especially without any clue.

Perhaps the teams can throw error, or simply ignore the .pfg if different net name is found and show warning.