[Plugin: AVH Extended Categories Widgets] Causes theme files to become uneditable

I downloaded and installed this plugin yesterday for three of my blogs. The result was that I was no longer to edit any of my theme pages via Appearance > Editor function in WP. I received the following error: Sorry, that file cannot be edited.

The error was the same regardless of what file I tried to edit and was the same across all three websites. As soon as I disabled the plugin, the error message disappeared and I was able to edit my theme files via the Appearance > Editor function.

https://www.ads-software.com/extend/plugins/extended-categories-widget/