Thank you for contacting us regarding this.
With 2012 Q3 we made a lot of performance improvements of RadRichTextBox. One of it was to avoid all unnecessary invoking of the MeasureAndArrange() method. In your code (following the steps to reproduce) MeasureAndArrangeInDefaultSize() method is called too often, which is irregular.
Indeed your code causes a crash, but we are interested what is its end purpose? Could you please elaborate more on your scenario so we could direct you to the most appropriate way to achieve it?
Looking forward to your reply.
the Telerik team