Merge pull request #4237 from starteam/master

Fix for empty imsx_messageIdentifier

AUTHORS

@@ -168,3 +168,4 @@ Juanan Pereira <juanan.pereira@ehu.es>
 Clinton Blackburn <cblackburn@edx.org>
 Dennis Jen <djen@edx.org>
 Filippo Valsorda <hi@filippo.io>
+Ivica Ceraj <ceraj@mit.edu>

common/lib/xmodule/xmodule/lti_module.py

@@ -694,7 +694,7 @@ oauth_consumer_key="", oauth_signature="frVp4JuvT1mVXlxktiAUjQ7%2F1cw%3D"'}
         parser = etree.XMLParser(ns_clean=True, recover=True, encoding='utf-8')
         root = etree.fromstring(data, parser=parser)
 
-        imsx_messageIdentifier = root.xpath("//def:imsx_messageIdentifier", namespaces=namespaces)[0].text
+        imsx_messageIdentifier = root.xpath("//def:imsx_messageIdentifier", namespaces=namespaces)[0].text or ''
         sourcedId = root.xpath("//def:sourcedId", namespaces=namespaces)[0].text
         score = root.xpath("//def:textString", namespaces=namespaces)[0].text
         action = root.xpath("//def:imsx_POXBody", namespaces=namespaces)[0].getchildren()[0].tag.replace('{'+lti_spec_namespace+'}', '')