If you are getting an "Invalid XAML" error in the Visual Studio designer after doing the "Convert to Telerik Application". This is usually caused by an old Windows Phone Controls namespace on the page (see the attached screenshot).
Look up to your xmlns declarations and find this one:
This is a relic from the Windows Phone 7 when you needed to use a Pivot or Panorama control. It is also the cause of your Invalid XAML error.
The proper namespace is now
However, in many newer app templates, this is already used for the xmlns:phone
declaration.. like this:
If you have the xmlns:phone
declaraiton, simply delete the bad line xmlns:controls
line. If you do not, then just change the assembly name from Microsoft.Phone.Controls
Everything should work now, happy coding!