I have a warning that shows up on the custom sidebar

I have a warning that shows up on the custom sidebar in Appearance > Widgets. I know this is just a warning but my client won't go for this being displayed as they're trying to edit their sidebar items. Can you assist? See attached.

The theme is a custom theme I've built and I did temporarily switch to the 2015 theme just to make sure its not theme related. I also switched off plugins one by one but still had the error.