Fill bucket similar colors crash if selecting multiple linked cells

[Kagoriyuki]

Pixelorama version: 0.11.3 [Linux 64-bit]

Issue description: Attempting to use the fill bucket to replace colors in the whole animation, I selected all cells and clicked which caused the crash. I'm able to reproduce while selecting just two cells as well.

Terminal output on crash:
Segmentation fault (core dumped)

Steps to reproduce:

1. Create multiple cells and link them in some way
2. Choose fill bucket and set the mode to similar colors
3. Select at least two of the linked cells and click a pixel

crash.mp4