added teardown step to main_window fixture

tests/test_gui.py

@@ -98,7 +98,15 @@ def main_window(qtbot, monkeypatch, use_temp_settings_file):
     # sys.excepthook = window.excepthook
     builtins.input = window.input
     window._toggle_reporting(True)
-    return window
+    yield window
+
+    app = QtWidgets.QApplication.instance()
+    if app is not None:
+        for w in app.topLevelWidgets():
+            w.close()
+        n = len(app.topLevelWidgets())
+        print(f"{n} widgets in the balance.")
+
 
 
 @pytest.fixture