In Sitefintiy 5.1 there was such a problem which appeared when the site used have more than one language on the site, this is fixed in sitefinity 5.2, the widget designer base class is expecting a single language. Please upgrade the site to an upper version (5.2 or upper) where the problem is fixed.
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items