Crash after deleting a texture

[BLOODWOLF333]

SLADE Version: 3.2.1
Operating System: Windows 10

Issue Details:
Had just exported Heretic's blue ice texture as a PNG to a folder on my computer, opened it in Gimp and desaturated the texture so that it actually looks like ice. Then imported it into my Heretic mod, deleted the old blue ice texture, and saved to received this error:

Relevant Files:
StackTrace.txt