Fix hang if exception is thrown during headless session app construction (#21688)

src/Headless/Avalonia.Headless/HeadlessUnitTestSession.cs

@@ -96,9 +96,18 @@ public sealed class HeadlessUnitTestSession : IDisposable, IAsyncDisposable
             using var globalCts = token.Register(s => ((CancellationTokenSource)s!).Cancel(), cts, true);
             using var localCts = cancellationToken.Register(s => ((CancellationTokenSource)s!).Cancel(), cts, true);
 
-            var application = _isolated
+            IDisposable application = null!;
-                ? EnsureIsolatedApplication()
+            try
-                : EnsureSharedApplication();
+            {
+                application = _isolated
+                    ? EnsureIsolatedApplication()
+                    : EnsureSharedApplication();
+            }
+            catch (Exception ex)
+            {
+                tcs.TrySetException(ex);
+                return; // Exit the dispatcher action if application initialization fails
+            }
 
             bool shouldCancel = false;
             Exception? caught = null;