Add protection against infinite startup.

testbed/tests/testbed.py

@@ -16,13 +16,26 @@
 def run_tests(app, cov, args, report_coverage, run_slow, running_in_ci):
     try:
         # Wait for the app's main window to be visible. Retrieving the actual main window
-        # will raise an exception until the app is actually initialized, so we check the
-        # underlying variable.
+        # will raise an exception until the app is actually initialized.
         print("Waiting for app to be ready for testing... ", end="", flush=True)
-        while app._main_window is app._UNDEFINED:
-            time.sleep(0.05)
-        while not app.main_window.visible:
+        i = 0
+        ready = False
+        while i < 100 and not ready:
+            try:
+                main_window = app.main_window
+                if main_window.visible:
+                    ready = True
+            except ValueError:
+                pass
+
             time.sleep(0.05)
+            i += 1
+
+        if not ready:
+            print("\nApp didn't display a main window.")
+            app.returncode = 1
+            return
+
         print("ready.")
         # Control the run speed of the test app.
         app.run_slow = run_slow