Description of the issue We’ve updated to SR4 and enabled SourceGen. The XAML compiled as expected with SourceGen enabled. However, a NullReferenceException is thrown at runtime. It appears to be caused by the OnPlatform XAML logic we have. When I add the following XAML:

<OnPlatform x:TypeArguments="View">
    <On Platform="WinUI">
        <Label Text="This is a label only visible on WinUI" />
    </On>
</OnPlatform>

the following exception is thrown:

System.NullReferenceException: Object reference not set to an instance of an object. at SourceGenRuntimeException.MainPage.InitializeComponent() in /Users/test/Desktop/SourceGenRuntimeException/obj/Generated/Microsoft.Maui.Controls.SourceGen/Microsoft.Maui.Controls.SourceGen.XamlGenerator/MainPage.xaml.xsg.cs:line 93 at SourceGenRuntimeException.MainPage..ctor() in /Users/test/Desktop/SourceGenRuntimeException/MainPage.xaml.cs:line 9 at SourceGenRuntimeException.App.CreateWindow(IActivationState activationState) in /Users/test/Desktop/SourceGenRuntimeException/App.xaml.cs:line 14 at Microsoft.Maui.Controls.Application.Microsoft.Maui.IApplication.CreateWindow(IActivationState activationState)

Steps to reproduce:

1. Run the attached sample project.
2. Compile the project and run it on a platform different from WinUI, as the OnPlatform example above targets WinUI. For instance, if the code is intended for Android, the issue will occur on all platforms except Android.

Expected Behavior The application should run as expected.

Actual Behavior A NullReferenceException is thrown.

Link to issue dotnet/maui#34074