Issue Summary

found a crash inducing bug when using the flipping inbetween feature. If you activate the flipping with ALT +Z and click on the timeline frame dial during the automatic motion Pencil2D will crash. It also happens when using the rolling ALT + X

Expected Results

Well ideally it shouldn't crash, but for a fix to be extra useful my recommendation is to stop the flipping action execution once the user clicks on the timeline dial numbers or "pull" the scrub header so the user regains command of the timeline immediately.

Steps to reproduce

1. Open Pencil2D file
2. Create a few keyframes
3. Go to the last keyframe and Press Alt + Z to initiate roll flipping
4. Quickly try moving the scrub header when the action is executing.
5. Your Pencil2D will hang and crash

System Information

• Pencil2D Version:

Version: 0.6.3
Operating System: Windows 7 SP 1 (6.1)
CPU Architecture: x86_64