fix: potential nullreferenceexception when accessing mainwindow.unityinstallationssource #219

Description

The code accesses MainWindow.unityInstallationsSource without null checking.
If this static property is null (e.g., during initialization or if scanning failed),
the assignment gridAvailableVersions.ItemsSource = MainWindow.unityInstallationsSource
will throw a NullReferenceException, crashing the window initialization.
This is a production crash risk.

Severity: medium
File: UnityLauncherPro/UpgradeWindow.xaml.cs

Expected Behavior

The code should handle this case properly to avoid unexpected errors or degraded quality.