FSL: FullSimLight crashes when the Plugin menu item is selected for Magnetic Field but no file is set

In FSL, FullSimLight crashes when the Plugin menu item is selected for the Magnetic Field but no file is set in the GUI:

 *** MAGNETIC FIELD SET FROM PLUGIN  *** 


libc++abi: terminating due to uncaught exception of type std::out_of_range: basic_string

### CAUGHT SIGNAL: 6 ### address: 0x1a1e3c724,  signal =  SIGABRT, value =    6, description = abort program (formerly SIGIOT). 

Instead of crashing, FSL should warn the user that they choose the "Plugin" menu item but didn't specify any file. FullSimLight should not be started until the user fixes the situation.

Ideally, the "FullSimLight" launch button should be greyed out until then, so to prevent the user to launch it and have it crashed.