Hi Maurice,

The reported error occurs when there is a problem with the javascript file in a project. The next step I would suggest is to inspect the index.html file inside the wwwroot folder in your Client project. The js you should have referenced should look like this:

(static assets)

<script src="_content/Telerik.UI.for.Blazor/js/telerik-blazor.js" defer></script>

or 

(CDN)

<script src="https://blazor.cdn.telerik.com/blazor/2.30.0/telerik-blazor.min.js" defer></script>

Please make sure you have one of them. 

I have to mark the current thread here as "declined" because it is not really a bug in the component. I suggest also checking our article for such javascript errors:

Javascript Errors article

If, after applying the ideas from above, you still get the same errors, please send us a small project that shows the error. This will allow us to investigate further and suggest next steps.

Thank you.

Regards,
Hristian Stefanov
Progress Telerik