If you remove the last span of a paragraph (on a measured document), using the inline collection, a NullReferenceException is thrown.

As a workaround: either make sure the manipulation happens on a non-measured document, or remove the span by using the RichTextEditor's API:

this.radRichTextEditor.Document.Selection.AddDocumentElementToSelection(span);
this.radRichTextEditor.Delete(false);