Fix possible crash navigating away from Dashboard

[krschau]

Summary of the pull request

If something terrible happens during Dashboard startup and the _initWidgetsCancellationTokenSource in not initialized, then cancelling it when we move away from the Dashboard can cause a crash. Check that the object is initialized before trying to use it.

References and relevant issues

Detailed description of the pull request / Additional comments

Validation steps performed

PR checklist

• Closes #xxx
• Tests added/passed
• Documentation updated

[DefaultRyan]

There's still a race condition because InitializeDashboard() does some amount of non-trivial work before initializing _initWidgetsCancellationTokenSource. So it's possible that we could see this sequence:

• OnLoadedAsync() called, InitializeDashboard() starts running, _initWidgetsCancellationTokenSource still null.
• OnUnloadedAsync() called, _initWidgetsCancellationTokenSource still null, so no call to Cancel().
• InitializeDashboard() finally reaches the point where it initializes _initWidgetsCancellationTokenSource, but because OnUnloadedAsync() is already finished, the cancellation never gets called.

Is there a reason we don't simply initialize the cancellation token in the constructor, so even in this case InitializeDashboard will eventually see the cancellation?