Handle None return from Docker network.containers property

Add defensive None check in InstanceManager.is_running() before
checking container membership in the network's container list.
The Docker SDK can return None from network.containers after reload()
under certain race conditions.

Author: Nils Bars

webapp/ref/core/instance.py

@@ -781,7 +781,7 @@ def is_running(self):
         # i.e., docker-compose down -> docker-compose up
         ssh_to_entry_network.reload()
         containers = ssh_to_entry_network.containers
-        if ssh_proxy_container not in containers:
+        if containers is None or ssh_proxy_container not in containers:
             return False
         if web_container not in containers:
             return False