I've added some better error output to the Designer to check what the problem is. Actually your activiti-File contains alot of "invisible" (invalid) pictogram data, e.g. double flows, etc.
If you for example see something like this
<flowElements xsi:type="bpmn2:EndEvent" id="endevent3" name="End" incoming="flow8 flow13Def"/>
it is most probably an error, as "flow8" is incoming but not outgoing. My corrected patch will produce error messages like:
An exception occurred while creating the BPMN 2.0 XML: No valid sourceRef ID for sequence flow 'flow8' found.
This helps indeed, but we are currently on our way to merge the activiti and the bpmn20.xml-files, therefore I would suggest you see that you remove the double flows and check your activiti-file for any invisible elements and remove them too. Afterwards the BPMN xml should be generated again.