Don't abortConnection() if the transport connection has already closed.

synapse/http/endpoint.py

@@ -113,10 +113,15 @@ def _time_things_out_maybe(self):
         if time.time() - self.last_request >= 2.5 * 60:
             self.abort()
             # Abort the underlying TLS connection. The abort() method calls
-            # loseConnection() on the underlying TLS connection which tries to
+            # loseConnection() on the TLS connection which tries to
             # shutdown the connection cleanly. We call abortConnection()
-            # since that will promptly close the underlying TCP connection.
-            self.transport.abortConnection()
+            # since that will promptly close the TLS connection.
+            #
+            # In Twisted >18.4; the TLS connection will be None if it has closed
+            # which will make abortConnection() throw. Check that the TLS connection
+            # is not None before trying to close it.
+            if self.transport.getHandle() is not None:
+                self.transport.abortConnection()
 
     def request(self, request):
         self.last_request = time.time()