[BUG] iframe is empty when calling view on a session that has closed

[axiomofjoy]

Describe the bug If the user has previously called phoenix.close_app and they subsequently call session.view(), we currently show an empty iframe. This is confusing, we should instead warn the user that no session is active and instruct them to relaunch the app.

To Reproduce

• call px.close_app() to close a previously launched session
• call session.view()

Expected behavior Warn the user that no session is active and provide them with instructions to relaunch Phoenix.

Recordings 36:10 of usability study

Environment (please complete the following information):

• OS: MacOS
• Notebook Runtime: Jupyter
• Browser: unknown
• Version: 0.0.6

[mikeldking]

I think this is semi-low priority since it requires checking if the session is active.

[axiomofjoy]

For now we could add a check for when the global session object is None. That will handle most cases unless the server stops for an unexpected reason.