diff --git a/src/plugins/platforms/cocoa/qcocoawindow.mm b/src/plugins/platforms/cocoa/qcocoawindow.mm
index e5c643751f6..85f7bbb5c1f 100644
--- a/src/plugins/platforms/cocoa/qcocoawindow.mm
+++ b/src/plugins/platforms/cocoa/qcocoawindow.mm
@@ -472,14 +472,10 @@ void QCocoaWindow::setVisible(bool visible)
     } else {
         // Window not visible, hide it
         if (isContentView()) {
-            if (eventDispatcher()->hasModalSession()) {
+            if (eventDispatcher()->hasModalSession())
                 eventDispatcher()->endModalSession(window());
-            } else {
-                if ([m_view.window isSheet]) {
-                    Q_ASSERT_X(parentCocoaWindow, "QCocoaWindow", "Window modal dialog has no transient parent.");
-                    [parentCocoaWindow->nativeWindow() endSheet:m_view.window];
-                }
-            }
+            else if ([m_view.window isSheet])
+                [m_view.window.sheetParent endSheet:m_view.window];
 
             // Note: We do not guard the order out by checking NSWindow.visible, as AppKit will
             // in some cases, such as when hiding the application, order out and make a window
diff --git a/tests/auto/widgets/dialogs/qdialog/tst_qdialog.cpp b/tests/auto/widgets/dialogs/qdialog/tst_qdialog.cpp
index 13f971f5f05..e970ea2810a 100644
--- a/tests/auto/widgets/dialogs/qdialog/tst_qdialog.cpp
+++ b/tests/auto/widgets/dialogs/qdialog/tst_qdialog.cpp
@@ -66,6 +66,8 @@ private slots:
     void deleteOnDone();
     void quitOnDone();
     void focusWidgetAfterOpen();
+
+    void closeParentOfVisibleDialog();
 };
 
 // Testing get/set functions
@@ -768,5 +770,23 @@ void tst_QDialog::focusWidgetAfterOpen()
     QCOMPARE(dialog.focusWidget(), static_cast<QWidget *>(pb2));
 }
 
+void tst_QDialog::closeParentOfVisibleDialog()
+{
+    QWidget widget;
+    widget.show();
+
+    // On macOS, this dialog becomes a sheet and triggered an assert when
+    // the widget it lived in was already gone by the time the sheet
+    // got closed. QTBUG-128302
+    QDialog dialog(&widget);
+    dialog.open();
+
+    QTimer::singleShot(0, &widget, [&]{
+        widget.close();
+    });
+
+    QTRY_VERIFY(!widget.isVisible());
+}
+
 QTEST_MAIN(tst_QDialog)
 #include "tst_qdialog.moc"