[develop] Errors about weakref upon LiSP close

[s0600204]

Expected Behavior

LiSP to close without error.

Actual Behavior

The following message is emitted multiple times: once for each Media cue that was created since LiSP's start:

Exception ignored in: <function WeakMethod.__new__.<locals>._cb at 0x7f8b385c9d90>
Traceback (most recent call last):
  File "/usr/lib/python3.7/weakref.py", line 55, in _cb
    if self._alive:
AttributeError: 'NoneType' object has no attribute '_alive'

Step to reproduce

1. Open LiSP from the command line,
2. Create a new showfile (doesn't matter which layout),
3. Add at least one Media Cue,
4. Remove at least one Media Cue,
5. Close LiSP,
6. Notice the error messages.

Investigation:

• Adding a Media Cue, then closing LiSP does not trigger the error messages.
• Having added a Media Cue, doing either of the following before closing does not trigger the error messages:
  • Moving the Media Cue around, or
  • Cloning the Media Cue in the Cart Layout.
• However, cloning a Media Cue in the List Layout does trigger the messages when LiSP closes.
• Errors do not occur if code is reverted to a state before c7520e6d.

[s0600204]

Tested, and this seems to be fixed. Thanks!