Checkbox value won't revert to a null value

[Sama34]

I was recently checking the Github repository to find if another issue I was having has already been fixed.

Apparently your issue (which I also experienced) has already been fixed:
https://github.com/zingaburga/XThreads-M...https://github.com/zingaburga/XThreads-MyBB-Plugin/commit/731f22777ff86f598d177cc1929dc6

I won't know if simply uploading the updating files is enough or if any update process is required because I haven't got the time to do it myself, but you could try and see if that works for you.