Explicitly check if windows exist when run as Dbus service

Author: gnunn1

source/gx/terminix/application.d

@@ -464,6 +464,7 @@ private:
         if (appWindow !is null) return appWindow;
 
         ListG list = getWindows();
+        if (list is null) return null;
         Window[] windows = list.toArray!(Window)();
         foreach(window; windows) {
             appWindow = cast(AppWindow) window;