Add check for DefaultRichEditBoxStyle Resource in UnitTests.UWP

UnitTests/UnitTests.UWP/App.xaml.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using Microsoft.VisualStudio.TestTools.UnitTesting.Logging;
 using System;
 using UnitTests.Extensions;
 using Windows.ApplicationModel;
@@ -90,6 +91,17 @@ protected override void OnLaunched(LaunchActivatedEventArgs e)
                     // TODO: Load state from previously suspended application
                 }
 
+                Logger.LogMessage("Looking for DefaultRichEditBoxStyle...");
+                if (!Resources.TryGetValue("DefaultRichEditBoxStyle", out var value))
+                {
+                    Logger.LogMessage("ERROR: Couldn't find DefaultRichEditBoxStyle in WinUI!");
+                    throw new ApplicationException("Couldn't find DefaultRichEditBoxStyle resource.");
+                }
+                else
+                {
+                    Logger.LogMessage("FOUND!");
+                }
+
                 // Place the frame in the current Window
                 Window.Current.Content = rootFrame;
             }