plugins/krunnner-integration: don't trust inputs from the dbus call

The desktop ID might not be valid, and then we'd crash.

src/plugins/krunner-integration/windowsrunnerinterface.cpp

@@ -207,6 +207,9 @@ void WindowsRunner::Run(const QString &id, const QString &actionId)
     if (action == ActivateDesktopAction) {
         QByteArray desktopId = objectId.toLocal8Bit();
         auto desktop = VirtualDesktopManager::self()->desktopForId(desktopId);
+        if (!desktop) {
+            return;
+        }
         VirtualDesktopManager::self()->setCurrent(desktop);
         return;
     }