Merge pull request #463 from iamvalya/master

fix: method flush() in iostream.py

Author: minrk

ipykernel/iostream.py

@@ -338,7 +338,7 @@ def flush(self):
 
         send will happen in the background thread
         """
-        if self.pub_thread.thread.is_alive():
+        if self.pub_thread and self.pub_thread.thread is not None and self.pub_thread.thread.is_alive():
             # request flush on the background thread
             self.pub_thread.schedule(self._flush)
             # wait for flush to actually get through, if we can.