Crash after PCell code exception is handled in Macro IDE

This is following the discussion here: https://www.klayout.de/forum/discussion/1842/coerce-parameters-impl#latest

To reproduce:
 * change a PCell's code to raise an exception in the "produce" implementation
 * open macro IDE, enable "stop of exception"
 * configure the PCell 
 
When the exception is caught, the macro IDE will pop up and the application will crash afterwards.