Crash when creating palette from sprite

When I create a palette from a sprite, Aseprite crashes a fraction of a second later. It happens often, but not every time: e.g. after recovering the edited file and creating a palette again, it does not crash.

System details

  • Macos 10.14.6 (Mojave), Macbook Air 11 inch, early 2014
  • Aseprite 1.2.17

Crash report on pastebin: Aseprite crash -