Fix: Ensure MainWindow instance is nullified on close

[seer-by-sentry]

Resolved / Related Issues

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• Closes FILES-APP-3C6

Steps used to test these changes

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1. The issue was that: SystemTrayIcon attempts to activate a disposed MainWindow instance when background running is disabled, causing a COMException.
2. Added an event handler for the Closed event of the MainWindow.
3. In the event handler, set the _Instance field to null when the main window is closed.

This fix was generated by Seer in Sentry, triggered by Yair. 👁️ Run ID: 502051

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