Fix: Prevent exception when setting SelectedTabIndex before MainPage is loaded

[seer-by-sentry]

Resolved / Related Issues

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• Closes FILES-APP-3NE

Steps used to test these changes

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1. The issue was that: AppModel's tab selection logic attempts to cast Frame.Content to MainPage while SplashScreenPage's Grid is still active, causing InvalidCastException.
2. Added a check to ensure that MainWindow.Instance.Content is a Frame and rootFrame.Content is a MainPage before accessing mainView.ViewModel.
3. This prevents a NullReferenceException that can occur when SelectedTabIndex is set before the MainPage is fully loaded.
4. This fix was generated by Seer in Sentry, triggered by Yair. 👁️ Run ID: 502085
5. Not quite right? Click here to continue debugging with Seer.

[hishitetsu]

It seems that they have made the fix based on old code but LGTM.
I wish they had fixed it like this from the beginning.