I do have the same error. It occured after an update to the current version of
the Microsoft.Toolkit.Uwp.UI.Controls Nuget package. Only the designer seems to be affected, the programm builds and runs normally.
Unfortunatly the control from the Toolkit i use (Radial Gauge) is an integral part of my application, and i did not find a way to replace it, so any help would be greatly appreciated.