kwinrules: Show error when detected window is unmanaged

BUG: 340477

kcmkwin/kwinrules/package/contents/ui/RulesEditor.qml

@@ -91,9 +91,10 @@ ScrollViewKCM {
             Layout.fillWidth: true
         }
         QQC2.Button {
+            id: detectButton
             text: i18n("Detect Window Properties")
             icon.name: "edit-find"
-            enabled: !propertySheet.sheetOpen
+            enabled: !propertySheet.sheetOpen && !errorSheet.sheetOpen
             onClicked: {
                 overlayModel.onlySuggestions = true;
                 rulesModel.detectWindowProperties(Math.max(delaySpin.value * 1000,
@@ -102,7 +103,7 @@ ScrollViewKCM {
         }
         QQC2.SpinBox {
             id: delaySpin
-            enabled: !propertySheet.sheetOpen
+            enabled: detectButton.enabled
             Layout.preferredWidth: Kirigami.Units.gridUnit * 8
             from: 0
             to: 30
@@ -120,6 +121,23 @@ ScrollViewKCM {
             overlayModel.onlySuggestions = true;
             propertySheet.sheetOpen = true;
         }
+        function onShowErrorMessage(message) {
+            errorLabel.text = message
+            errorSheet.open()
+        }
+    }
+
+    Kirigami.OverlaySheet {
+        id: errorSheet
+        header: Kirigami.Heading {
+            text: i18n("Error")
+        }
+        Kirigami.Heading {
+            id: errorLabel
+            level: 3
+            wrapMode: Text.WordWrap
+            Layout.fillWidth: true
+        }
     }
 
     Kirigami.OverlaySheet {

kcmkwin/kwinrules/rulesmodel.cpp

@@ -829,6 +829,9 @@ void RulesModel::selectX11Window()
                 QDBusPendingReply<QVariantMap> reply = *self;
                 self->deleteLater();
                 if (!reply.isValid()) {
+                    if (reply.error().name() == QLatin1String("org.kde.KWin.Error.InvalidWindow")) {
+                        emit showErrorMessage(i18n("Could not detect window properties. The window is not managed by KWin."));
+                    }
                     return;
                 }
                 const QVariantMap windowInfo = reply.value();

kcmkwin/kwinrules/rulesmodel.h

@@ -81,7 +81,9 @@ public:
 signals:
     void descriptionChanged();
     void warningMessageChanged();
+
     void showSuggestions();
+    void showErrorMessage(const QString &message);
 
     void virtualDesktopsUpdated();