RunPlanButton should deal with no instrument session selected

If no instrument session is selected, a plan cannot be run. A default should not be provided, as this will likely only cause further errors downstream.

The RunPlanButton should allow a `null` instrumentSession, and refuse to submit the plan, raising an error and notifying the user that no session is selected.

Locate the RunPlanButton component and follow its plan-submission path. Check how an absent instrument session is represented and how user errors are notified. Done means no default session is supplied, submission is refused when the session is null, and the user is told that no session is selected.

Written by the indexing model from the issue text.