[CRASH] NoneType object is not subscriptable

[EliteMasterEric]

Traceback (most recent call last):
  File "D:\Programming\Game Modding\Friday Night Funkin\Spritesheets\FnF-Spritesheet-and-XML-Maker\src\xmlpngUI.py", line 306, in generate_xml
    statuscode, errmsg = xmlpngengine.make_png_xml(
  File "D:\Programming\Game Modding\Friday Night Funkin\Spritesheets\FnF-Spritesheet-and-XML-Maker\src\xmlpngengine.py", line 166, in make_png_xml
    final_img_width, final_img_height, max_heights = calculate_final_size(existing_img_dict, imlist, num_cols, clip, reuse_sprites_level)
  File "D:\Programming\Game Modding\Friday Night Funkin\Spritesheets\FnF-Spritesheet-and-XML-Maker\src\xmlpngengine.py", line 99, in calculate_final_size
    widths.append(box[2] - box[0] + 4)
TypeError: 'NoneType' object is not subscriptable

[EliteMasterEric]

I've determined the cause of the issue. If the frame contains no pixels (as an example, the animation from the Tricky Mod where Expurgation Tricky enters has several empty frames at the start), box will be null.

I'll make a pull request for this myself since it shouldn't be too hard.