diff --git a/layers.cpp b/layers.cpp
--- a/layers.cpp
+++ b/layers.cpp
@@ -693,7 +693,7 @@
 // Returns all windows in their stacking order on the root window.
 ToplevelList Workspace::xStackingOrder() const
 {
-    if (m_xStackingQueryTree) {
+    if (m_xStackingDirty) {
         const_cast<Workspace*>(this)->updateXStackingOrder();
     }
     return x_stacking;
@@ -707,7 +707,7 @@
     foreach (Toplevel * c, stacking_order)
     x_stacking.append(c);
 
-    if (!tree->isNull()) {
+    if (tree && !tree->isNull()) {
         xcb_window_t *windows = tree->children();
         const auto count = tree->data()->children_len;
         int foundUnmanagedCount = unmanaged.count();
@@ -735,6 +735,7 @@
             }
         }
     }
+    m_xStackingDirty = false;
 }
 
 //*******************************
diff --git a/workspace.h b/workspace.h
--- a/workspace.h
+++ b/workspace.h
@@ -565,6 +565,7 @@
     bool force_restacking;
     ToplevelList x_stacking; // From XQueryTree()
     std::unique_ptr<Xcb::Tree> m_xStackingQueryTree;
+    bool m_xStackingDirty = false;
     QList<AbstractClient*> should_get_focus; // Last is most recent
     QList<AbstractClient*> attention_chain;
 
diff --git a/workspace.cpp b/workspace.cpp
--- a/workspace.cpp
+++ b/workspace.cpp
@@ -1696,7 +1696,10 @@
 
 void Workspace::markXStackingOrderAsDirty()
 {
-    m_xStackingQueryTree.reset(new Xcb::Tree(rootWindow()));
+    m_xStackingDirty = true;
+    if (kwinApp()->x11Connection()) {
+        m_xStackingQueryTree.reset(new Xcb::Tree(kwinApp()->x11RootWindow()));
+    }
 }
 
 void Workspace::setWasUserInteraction()