Changing the IsEnable property of the document history should clear the undo and redo. Executing undo or redo over a document with changed structure will result in an invalid document.

Steps to reproduce:
1. Add text to the RadRichTextBox.
2. Disable the document history.

Expected: The undo and redo are cleared.
Actual: the undo is not cleared and the MultipleUndoControl contains all changes.