title: JavaFX TabPane,Tab.setContent() doesn't work
body: <p>The problem is that i want to transfer an item between a <code>GridPane</code> and a <code>Tab</code>
  so i am adding the item to the tab first.</p>
  
  <p>Then i am adding the same item to the <strong>GridPane</strong>.Its works well until here.But then i want to add the item back to the tab and i am using  </p>
  
  <pre><code>tab.setContent(item);
  </code></pre>
  
  <p>But it doesn't work.Neither the item is removed from the GridPane neither the item is added to the tab as content.Why is this happening??</p>
  
  <blockquote>
    <p>The <strong>.java</strong> file:</p>
  </blockquote>
  
  <pre><code>package pack;
  
  import java.io.IOException;
  import java.net.URL;
  import java.util.ResourceBundle;
  
  import javafx.application.Application;
  import javafx.fxml.FXML;
  import javafx.fxml.FXMLLoader;
  import javafx.fxml.Initializable;
  import javafx.scene.Scene;
  import javafx.scene.control.Button;
  import javafx.scene.control.Tab;
  import javafx.scene.layout.BorderPane;
  import javafx.scene.layout.GridPane;
  import javafx.stage.Stage;
  
  public class Starter extends Application {
  
      @Override
      public void start(Stage primaryStage) throws Exception {
          primaryStage.setScene(new Scene(new UI()));
          primaryStage.centerOnScreen();
          primaryStage.show();
      }
  
      public static void main(String[] args) {
          launch(args);
      }
  
      /**
       * This class contains the graphical interface
       * 
       * @author Alexander
       *
       */
      public class UI extends BorderPane implements Initializable {
  
          @FXML
          private Tab tab;
  
          @FXML
          private Button item;
  
          @FXML
          private GridPane gridPane;
  
          @FXML
          private Button transferButton;
  
          boolean onGridPane = false;
  
          /**
           * Constructor
           */
          public UI() {
              FXMLLoader loader = new FXMLLoader(getClass().getResource("UI.fxml"));
              loader.setController(this);
              loader.setRoot(this);
  
              try {
                  loader.load();
              } catch (IOException e) {
                  e.printStackTrace();
              }
          }
  
          @Override
          public void initialize(URL location, ResourceBundle resources) {
              System.out.println("Main Controller has been initialized....");
  
              transferButton.setOnAction(a -&gt; {
                  if (!onGridPane) {
                      gridPane.add(item, 0, 0);
                      onGridPane = true;
                      transferButton.setText("&lt;-Move to Tab");
                  } else {
                      tab.setContent(item);
                      onGridPane = false;
                      transferButton.setText("Move to GridPane-&gt;");
                  }
              });
          }
  
      }
  
  }
  </code></pre>
  
  <blockquote>
    <p>And the <strong>.fxml</strong> file:</p>
  </blockquote>
  
  <pre><code>&lt;?xml version="1.0" encoding="UTF-8"?&gt;
  
  &lt;?import javafx.geometry.Insets?&gt;
  &lt;?import javafx.scene.control.Button?&gt;
  &lt;?import javafx.scene.control.Label?&gt;
  &lt;?import javafx.scene.control.Tab?&gt;
  &lt;?import javafx.scene.control.TabPane?&gt;
  &lt;?import javafx.scene.layout.BorderPane?&gt;
  &lt;?import javafx.scene.layout.ColumnConstraints?&gt;
  &lt;?import javafx.scene.layout.GridPane?&gt;
  &lt;?import javafx.scene.layout.RowConstraints?&gt;
  &lt;?import javafx.scene.text.Font?&gt;
  
  &lt;fx:root prefHeight="322.0" prefWidth="538.0" type="BorderPane" xmlns="http://javafx.com/javafx/8.0.60" xmlns:fx="http://javafx.com/fxml/1"&gt;
     &lt;top&gt;
        &lt;TabPane prefHeight="141.0" prefWidth="538.0" style="-fx-border-color: red;" tabClosingPolicy="UNAVAILABLE" BorderPane.alignment="CENTER"&gt;
          &lt;tabs&gt;
            &lt;Tab fx:id="tab" text="Tab"&gt;
                 &lt;content&gt;
                    &lt;Button fx:id="item" mnemonicParsing="false" text="Moving Item" /&gt;
                 &lt;/content&gt;
            &lt;/Tab&gt;
          &lt;/tabs&gt;
        &lt;/TabPane&gt;
     &lt;/top&gt;
     &lt;bottom&gt;
        &lt;GridPane fx:id="gridPane" gridLinesVisible="true" style="-fx-border-color: red;" BorderPane.alignment="CENTER"&gt;
          &lt;columnConstraints&gt;
            &lt;ColumnConstraints hgrow="SOMETIMES" minWidth="10.0" prefWidth="100.0" /&gt;
          &lt;/columnConstraints&gt;
          &lt;rowConstraints&gt;
            &lt;RowConstraints minHeight="10.0" prefHeight="30.0" vgrow="SOMETIMES" /&gt;
          &lt;/rowConstraints&gt;
        &lt;/GridPane&gt;
     &lt;/bottom&gt;
     &lt;right&gt;
        &lt;Button fx:id="transferButton" mnemonicParsing="false" text="Move to GridPane-&amp;gt;" BorderPane.alignment="CENTER"&gt;
           &lt;font&gt;
              &lt;Font size="16.0" /&gt;
           &lt;/font&gt;&lt;/Button&gt;
     &lt;/right&gt;
     &lt;padding&gt;
        &lt;Insets bottom="5.0" left="5.0" right="5.0" top="5.0" /&gt;
     &lt;/padding&gt;
     &lt;center&gt;
        &lt;Label prefHeight="82.0" prefWidth="340.0" style="-fx-font-weight: bold; -fx-font-size: 15;" text="Above is a [TabPane] and Below is a [GridPane]" BorderPane.alignment="CENTER" /&gt;
     &lt;/center&gt;
  &lt;/fx:root&gt;
  </code></pre>