Node.lookup が一部機能しない

<?xml version="1.0" encoding="UTF-8"?>

<?import java.lang.*?>

<?import java.net.*?>

<?import java.util.*?>

<?import javafx.geometry.*?>

<?import javafx.scene.control.*?>

<?import javafx.scene.image.*?>

<?import javafx.scene.layout.*?>

<?import javafx.scene.paint.*?>

<?import javafx.scene.text.*?>

<BorderPane maxHeight="-Infinity" maxWidth="-Infinity" minHeight="-Infinity" minWidth="-Infinity" prefHeight="371.0" prefWidth="505.0" stylesheets="@SimpleTextView.css" xmlns="http://javafx.com/javafx/8" xmlns:fx="http://javafx.com/fxml/1">

   <center>

      <ScrollPane fitToHeight="true" fitToWidth="true" prefHeight="511.0" prefWidth="600.0" BorderPane.alignment="CENTER">

         <content>

           <TextArea id="source_text" fx:id="source_text" prefHeight="200.0" prefWidth="200.0">

               <font>

                  <Font name="SansSerif Regular" size="12.0" />

               </font>

           </TextArea>

         </content>

      </ScrollPane>

   </center>

   <top>

      <Label fx:id="main_label" alignment="CENTER" contentDisplay="CENTER" layoutX="0.5" layoutY="0.5" nodeOrientation="LEFT_TO_RIGHT" prefHeight="30.0" prefWidth="149.0" snapToPixel="false" style="" styleClass="button" text="View source" textAlignment="CENTER" BorderPane.alignment="CENTER">

         <font>

            <Font name="Lucida Sans Demibold" size="16.0" />

         </font>

      </Label>

   </top>

</BorderPane>

BorderPane panel = FXMLLoader.load(getClass().getResource("SimpleTextView.fxml"));