Import as Spritesheet fails with GIFs

[juliandescottes]

Importing a single frame GIF as a spritesheet fails.

Steps to reproduce the bug

1. Download the attached GIF
2. go to piskelapp.com
3. open import panel, select browse images
4. import the GIF as a spritesheep, select any frame size
   4 click import

=> nothing happens, remain stuck on the dialog.

Environment details

• operating system: OSX
• browser: Chrome