Hi @Gideon Engelbrecht ,
Welcome to Microsoft Q&A!
In VS2019. I can reproduce your issue in WPF application (targeting .NET Core and .NET Framework) with VB.
Also tested it in VS2022, I found the RadialProgressBar control can be added into toolbox in .NET Framework project. But the same error occurs in .NET Core project.
I suggest you report this issue to product team in Developer Community.
Meanwhile, for the NuGet package XamlRadialProgressBar, it is personally owned by panthernet and the latest version is updated in 4/16/2020. I suppose it may be a little too old. I suggest you can also report to it’s owner.
Sincerely,
Anna
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