In the latest Q3 release - 2011.3.1116 we introduced a new functionality - styles. Before, the StyleDefinition objects were only used to set local values of the properties such as FontSize, FontWeight, etc. With the new implementation, setting a style with value null will be a problem.
Overall, you have two solutions:
1. Create a StyleDefinition, set some of its properties and pass it as an argument.
2. Use the method or RadRichTextBox.
There are code-snippets with both approaches and a description of the difference in this article. The main difference is that the undo/redo stack will be cleared if you use the Insert method of RadDocument as you have been doing by now.
the Telerik team
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