Hi Paul,
I've had a look at this. I've determined that the problem is caused by the polylang plugin. The polylang plugin is trying to emulate some of the functionality of the WPML plugin but it doesn't do a complete job of it. This was causing a problem with the WP Views Filter widget and was causing a problem on the customize page and the widgets page.
We'll update the code in our plugin to get around this problem with the polylang plugin. I've patched the plugin code in your sandbox.
If you want better multilingual support for our plugin you might consider using our WPML plugin.
Best regards,
Bruce