RadBusyIndicator throw error when theme is Metro and is not set as an Application theme. Message=Cannot access Freezable 'System.Windows.Media.SolidColorBrush' across threads because it cannot be frozen.
Whenever you are running metro styled telerik components you must invoke:
So the shared color resources we bind our controls to could be frozen and become thread safe. The above line could be placed in the OnStartup in your example and it would work.