TreeItem cannot resolve onmouseclicked

[ASchmidt84]

Very strange behavior.

package view.helperFunctions

import scalafx.scene.Node
import scalafx.scene.control.TreeItem
import scalafx.scene.input.MouseEvent
import scalafx.scene.control.{DatePicker, TitledPane, TreeItem, TreeView}

/**
  * Created by André Schmidt on 18.07.2017.
  */
class TEST {
  import scala.language.postfixOps
  import scalafx.Includes._

  val a = new TreeView[String](){
    onMouseClicked = (_: MouseEvent) => {}
  }

  val b = new TreeItem(""){
    onMouseClicked = (m: MouseEvent) => {}
  }

}

All looks fine. But at val b my idea tells me "cannot resolve symbol onMouseClicked"
In another file this is working!

private val dossierNode = new TreeItem[String](Messages("dashboard.dossier.title")){
    val newDossier = new TreeItem[String](Messages("generic.new")){
      onMouseClicked = (me: MouseEvent) => {
        if(me.clickCount == 2){
          //
        }
      }
    }

    children += newDossier
  }

package view.pane.dashboard

Anyone an idea?