fix error if page is deleted and no longer available in menu (#25)

Author: de-raaf-media

src/app/Models/MenuItem.php

@@ -67,7 +67,9 @@ public function url()
                 break;
 
             default: //page_link
-                return url($this->page->slug);
+                if ($this->page) {
+                    return url($this->page->slug);
+                }
                 break;
         }
     }