diff --git a/src/platformtheme/x11integration.cpp b/src/platformtheme/x11integration.cpp
--- a/src/platformtheme/x11integration.cpp
+++ b/src/platformtheme/x11integration.cpp
@@ -85,7 +85,7 @@
 
 void X11Integration::installColorScheme(QWindow *w)
 {
-    if (!w->isTopLevel()) {
+    if (!w->isTopLevel() || !w->handle() /* e.g. WebEngine's QQuickWindow */) {
         return;
     }
     static xcb_atom_t atom = XCB_ATOM_NONE;