Hello bscarlavai ,
I think I found the problem, this exception is thrown on the function that retrieves the tabs names.
I debugged the code and it was on Notes tab where there is no "name" attribute in the XML form.
This is the code :
from elem in form.Descendants("tabs") .Descendants("tab")
I added where condition ( where elem.Attribute("name") !=null ) and it works now.