Scala forced me to do casting!: 1.Demonstration : Error Msg

Error:(20, 17) overloaded method value addAll with alternatives:
  (x$1: javafx.scene.Node*)Boolean <and>
  (x$1: java.util.Collection[_ <: javafx.scene.Node])Boolean
 cannot be applied to (scalafx.scene.layout.StackPane)
        content.addAll(new StackPane{content = new Text("Demo")})
                ^