Upgrade null safety example (#748)

null_safety/null_unsafe_app/test/widget_test.dart

@@ -0,0 +1,15 @@
+// This is a basic Flutter widget test.
+//
+// To perform an interaction with a widget in your test, use the WidgetTester
+// utility that Flutter provides. For example, you can send tap and scroll
+// gestures. You can also use WidgetTester to find child widgets in the widget
+// tree, read text, and verify that the values of widget properties are correct.
+
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('Rendering the page throws an exception',
+      (WidgetTester tester) async {
+    // Do nothing, running the app throws an exception on widget render.
+  });
+}