Generally, the document which is loaded by default consists of "<p>&nbsp;</p>"  (or "<p></p" depending on the parameter "NBSPFILL").

This problem usually occurs, when the ondataloaded function is used and an empty document is loaded into edit-on Pro. Then, the empty paragraph is replaced by the empty content of the document. You can avoid this by checking the document, which should be loaded into edit-on Pro for emptiness: If it's empty, you should perform either no action or load "<p>&nbsp;</p>".