I have some problems reproducing the error. I created a sample application with RadBusyIndicator only and the style you provided but it is working fine at my side and no error is thrown. I have attached the project I used. Do you get the same error with it? Also some info what is the error would be of help.
the Telerik team
Do you want to have your say when we set our development plans?
Do you want to know when a feature you care about is added or when a bug fixed?
Telerik Public Issue Tracking
system and vote to affect the priority of the items