Fix treeview stuck with only 'select tenant' node

[peterbom]

Addresses #680

This is just to fix the observed behaviour that:

1. The treeview can display a node saying 'select tenant' when in fact there's only one tenant to choose from; and
2. The 'select tenant' command currently doesn't do anything if there is only a single tenant to choose from.

There are some additional changes we can make to the session provider API that should make this kind of defect less likely. I'll address those in a separate issue and PR.

[Tatsinnit]

👍 looks good to me, very specific to the scenario. I could test it latter tonight. (Due to specific nature I believe this might have got missed from the tests done around this)

[peterbom]

👍 looks good to me, very specific to the scenario. I could test it latter tonight. (Due to specific nature I believe this might have got missed from the tests done around this)

Thank you. I agree we wouldn't have tested this scenario - I'm actually unable to come up with concrete repro steps, as it involves simply not using the extension for an extended number of days (I don't know how many).