If content with DocumentLists and ListStyles respectively, is copied from Microsoft Word and then pasted in RadRichTextBox, ArgumentNullException is thrown while the lists from the DocumentFragment are fixed (FixListStyles() method). 

The exception is caught and thus, the content from the clipboard is not inserted during the copy/paste operation. In some cases, when the content is pasted again, the content is pasted.

Workaround: Replace the fragment in the clipboard with one that is already "fixed". See the attached MainWindow.cs.txt