Merge remote-tracking branch 'upstream/master' into Update_comms_matplotlib

Author: Quentin Peter

.travis.yml

@@ -1,11 +1,10 @@
 language: python
 python:
   - "nightly"
-  - "3.8-dev"
+  - 3.8
   - 3.7
   - 3.6
   - 3.5
-  - 3.4
 sudo: false
 dist: xenial
 install:
@@ -41,10 +40,6 @@ after_success:
   - codecov
 matrix:
   include:
-    - python: 3.5
-      env:
-        - TORNADO="4.5.*"
-        - IPYTHON=master
     - python: 3.6
       env:
         - IPYTHON=master

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.