jupyterhub_hub   | [I 2020-04-02 10:10:50.392 JupyterHub dockerspawner:706] Found existing container jupyter-tester2 (id: 236a587)
jupyterhub_hub   | [I 2020-04-02 10:10:50.392 JupyterHub dockerspawner:721] Starting container jupyter-tester2 (id: 236a587)
jupyterhub_hub   | [E 2020-04-02 10:10:50.727 JupyterHub user:477] Unhandled error starting tester2's server: The 'ip' trait of a Server instance must be a unicode string, but a value of None <class 'NoneType'> was specified.