Fixes issue where Alert crashes (#11168)

* Fixes issue where Alert crashes

There are edge cases where an Alert may be shown while the app is
exiting. Generally, when the app is exiting, apps will have set the
XamlRoot instance on XamlUIService to null. In this case, we may still
attempt to show the alert on a XAML Island with a null XamlRoot
instance, which would cause a crash.

This change just ensures that, if we are in a XAML Island and we don't
have a valid XamlRoot to attach to the ContentDialog, we just treat the
alert as dismissed.

* Change files

change/react-native-windows-7857ea29-339a-42e2-958b-383fecd2ce0e.json

@@ -0,0 +1,7 @@
+{
+  "type": "prerelease",
+  "comment": "Fixes issue where Alert crashes",
+  "packageName": "react-native-windows",
+  "email": "erozell@outlook.com",
+  "dependentChangeType": "patch"
+}

vnext/Microsoft.ReactNative/Modules/AlertModule.cpp

@@ -103,6 +103,13 @@ void Alert::ProcessPendingAlertRequestsXaml() noexcept {
       });
 
       dialog.Closed([=](auto &&, auto &&) { xamlRoot.Changed(rootChangedToken); });
+    } else if (IsXamlIsland()) {
+      // We cannot show a ContentDialog in a XAML Island unless it is assigned a
+      // XamlRoot instance. In such cases, we just treat the alert as dismissed.
+      jsDispatcher.Post([result, this] { result(m_constants.dismissed, m_constants.buttonNeutral); });
+      pendingAlerts.pop();
+      ProcessPendingAlertRequests();
+      return;
     }
   }