An unhandled exception was thrown by the 'Execute' method

[klaudia-glowacka]

Describe the bug

When running UI tests with MSTest in a .NET 9 project, the test initialization fails with the following error:
TestMethod 2 An unhandled exception was thrown by the 'Execute' method. Please report this error to the author of the attribute 'Microsoft.VisualStudio.TestTools.UnitTesting.AppContainer.UITestMethodAttribute'. System.InvalidOperationException: UITestMethodAttribute.DispatcherQueue should not be null. To use UITestMethodAttribute within a WinUI Desktop App, remember to set the static UITestMethodAttribute.DispatcherQueue during the test initialization. at Microsoft.VisualStudio.TestTools.UnitTesting.AppContainer.UITestMethodAttribute.ExecuteAsync(ITestMethod testMethod) in //src/TestFramework/TestFramework.Extensions/Attributes/WinUI_UITestMethodAttribute.cs:line 48 at Microsoft.VisualStudio.TestPlatform.MSTest.TestAdapter.Execution.TestMethodRunner.<>c__DisplayClass11_0.<b__0>d.MoveNext() in //src/Adapter/MSTestAdapter.PlatformServices/Execution/TestMethodRunner.cs:line 409 --- End of stack trace from previous location --- at Microsoft.VisualStudio.TestPlatform.MSTest.TestAdapter.Execution.TestMethodRunner.ExecuteTestAsync(TestMethodInfo testMethodInfo) in /_/src/Adapter/MSTestAdapter.PlatformServices/Execution/TestMethodRunner.cs:line 418

Version used

MSTest" Version="4.1.0"
MSTest.TestAdapter" Version="4.1.0"
MSTest.TestFramework" Version="4.1.0"
Microsoft.TestPlatform.TestHost" Version="18.3.0" />
Microsoft.Windows.SDK.BuildTools" Version="10.0.26100.7705"
Microsoft.WindowsAppSDK" Version="1.8.260209005"

Steps To Reproduce

1. Create a UI test project targeting .NET 9 using MSTest.
2. Run the tests using Visual Studio or the command line.
3. Observe that it throws an exception.

Expected behavior

The test app should launch, and UITestMethodAttribute.DispatcherQueue should be initialized before tests run, allowing UI tests to execute.

Actual behavior

The test app does not appear to launch or initialize the dispatcher queue, causing the initialization to fail and all tests to abort.

Additional context

[Youssef1313]

@nohwnd I haven't double checked, but this could be an issue on VSTest side related to deployment? I think we have a DC ticket claiming something broken around WinUI and is reported as VS regression. Can you double check please?