Fix crash when dragging an image and toggling on crop mode

[DarkDefender]

crop_mode_event_start would be set to None and thus crash the program. Add a check for this.

Note that going into crop mode can crash in different ways without #113. So this only fixes the crash I was able to reproduce with #113 applied.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 97.94%. Comparing base (6e3bc35) to head (e02ccc6). Report is 3 commits behind head on main.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #114 +/- ## ======================================= Coverage 97.94% 97.94% ======================================= Files 33 33 Lines 4375 4375 ======================================= Hits 4285 4285 Misses 90 90 ```

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[rbreu]

Thank a lot!