End-Advance crashes if the target is deleted

[robmcl]

- Cue #1 End-Advance target is set to Cue#2
- Delete Cue#2
- Cue #1 still shows End-Advance as the action but of course the Target is now blank.
- Attempt to make any change to Cue#1 and Accept (without noticing that the EA-Target is blank) will crash the program.
v214

[David]

OK, that issue would happen with any Cue Advance Action (other than No Action).

I've found the cause and modified the code (for release v215) to ignore the blank Cue Advance Target when accepting changes in Cue Properties.

[robmcl]

Thank you!