A possible reason for this exception that comes to mind is if the Generic.xaml dictionary is not merged in the resources of the application as stated here: Add Styles to the DataGrid.
<Application.Resources><ResourceDictionary><ResourceDictionary.MergedDictionaries><XamlControlsResourcesxmlns="using:Microsoft.UI.Xaml.Controls" /><ResourceDictionarySource="ms-appx:///Telerik.WinUI.Controls/Themes/Generic.xaml"/><!-- Other merged dictionaries here --></ResourceDictionary.MergedDictionaries><!-- Other app resources here --></ResourceDictionary></Application.Resources>
If you have not done so, can you please try adding this dictionary and check if the exception is still thrown?
If that is indeed the case, please share the full Message of the exception and ideally - a small sample project which demonstrates it so that I can provide further guidance on the matter.
Regards,
Dilyan Traykov
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Thank you for the provided project. Indeed, upon running it, I was also presented with an exception.
I have traced its source and can confirm that this seems to be a bug at our end. I have thus logged the following bug report regarding it on your behalf: Exception is thrown when creating an unpackaged application. As a thank you for your help in addressing this, I've also awarded you some Telerik points.
Please follow the item to get notified about any changes in its status. I hope we can resolve this issue soon.