diff --git a/src/kmessagebox.cpp b/src/kmessagebox.cpp
--- a/src/kmessagebox.cpp
+++ b/src/kmessagebox.cpp
@@ -141,6 +141,10 @@
     Q_ASSERT(subWindow);
 
     QWindow *mainWindow = QWindow::fromWinId(mainWindowId);
+    if (!mainWindow) {
+        // foreign windows not supported on all platforms
+        return;
+    }
     // mainWindow is not the child of any object, so make sure it gets deleted at some point
     QObject::connect(subWidget, &QObject::destroyed, mainWindow, &QObject::deleteLater);
     subWindow->setTransientParent(mainWindow);