Hi there
Thanks for your report!
Unfortunately I can’t reproduce this at the moment. Does this still happen with all other plugins deactivated?
Anyway, we’ll make sure to harden the plugin in such a way that this error message shouldn’t appear anymore.
You can track progress here: https://github.com/wearerequired/WP-Widget-Disable/issues/28
Cheers
Pascal
Just a quick heads up that the fix I mentioned above is part of the 1.9.0 update of WP Widget Disable.
Happy updating!