Editor locks up when template string is empty

[grymmy]

Describe the bug
Bitmap fails to open when the template string is empty (i.e. when first right the legend for the sprite). After opening and clicking the editor it freezes and you need to reload

To Reproduce
Please see slack thread for additional details.

Expected behavior
The editor window should not freeze.

Actual behavior
The editor window freezes.

Screenshots

[JosiasAurel]

steps to reproduce

• open the sprig editor
• change the theme value in localstorage to something other than ligh/dark/busker
• reload the page
• open the bitmap editor and try to edit or close the bitmap editor
  • observe that it throws errors in the console and bitmap editor does not close