diff --git a/gui/qt/util.py b/gui/qt/util.py
index 97a1ab36..4b549ec4 100644
--- a/gui/qt/util.py
+++ b/gui/qt/util.py
@@ -148,9 +148,10 @@ class CancelButton(QPushButton):
 class MessageBoxMixin(object):
     def top_level_window(self, window=None):
         window = window or self
+        classes = (WindowModalDialog, QMessageBox)
         for n, child in enumerate(window.children()):
             # Test for visibility as old closed dialogs may not be GC-ed
-            if isinstance(child, WindowModalDialog) and child.isVisible():
+            if isinstance(child, classes) and child.isVisible():
                 return self.top_level_window(child)
         return window