Initialize cancellation token in DashboardView constructor

[krschau]

Summary of the pull request

#3619 (comment)
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.

Instead, we should initialize the cancellation token in the constructor, so even in this case InitializeDashboard will eventually see the cancellation.

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Detailed description of the pull request / Additional comments

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PR checklist

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

[DefaultRyan]

Minor nonblocking nit for simplifying initialization