Prevent None from being added to the schedule from a timing issue.

sleekxmpp/xmlstream/scheduler.py

@@ -172,6 +172,7 @@ class Scheduler(object):
                 else:
                     updated = True
                     self.schedule_lock.acquire()
+                    if newtask is not None:
                         self.schedule.append(newtask)
                 finally:
                     if updated: