RootLayout.fxml

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

<?import javafx.scene.control.Button?>
<?import javafx.scene.control.Menu?>
<?import javafx.scene.control.MenuBar?>
<?import javafx.scene.control.MenuItem?>
<?import javafx.scene.control.Tab?>
<?import javafx.scene.control.TabPane?>
<?import javafx.scene.control.ToolBar?>
<?import javafx.scene.layout.AnchorPane?>
<?import javafx.scene.layout.BorderPane?>
<?import javafx.scene.layout.VBox?>

<BorderPane id="BorderPane" prefHeight="400.0" prefWidth="600.0" xmlns="http://javafx.com/javafx/8.0.72" xmlns:fx="http://javafx.com/fxml/1" fx:controller="org.nilsding.j4b.hue02.kurssystem.view.RootLayoutController">
   <center>
      <TabPane fx:id="tabPane" prefHeight="200.0" prefWidth="200.0" tabClosingPolicy="UNAVAILABLE" BorderPane.alignment="CENTER">
        <tabs>
          <Tab fx:id="tabKurse" closable="false" text="Kurse" />
          <Tab fx:id="tabDozenten" closable="false" text="Dozenten" />
        </tabs>
      </TabPane>
   </center>
   <top>
      <VBox BorderPane.alignment="CENTER">
         <children>
              <!-- useSystemMenuBar => use global menu bar on OS X; this fucks up SceneBuilder's menu bar though -->
              <MenuBar useSystemMenuBar="true">
                  <menus>
                      <Menu mnemonicParsing="false" text="File">
                          <items>
                              <MenuItem mnemonicParsing="false" onAction="#handleQuit" text="Quit" />
                          </items>
                      </Menu>
                      <Menu mnemonicParsing="false" text="Help">
                          <items>
                              <MenuItem mnemonicParsing="false" onAction="#handleAbout" text="About" />
                          </items>
                      </Menu>
                  </menus>
              </MenuBar>
            <ToolBar prefHeight="40.0" prefWidth="200.0">
              <items>
                  <Button fx:id="buttonRefresh" mnemonicParsing="false" onAction="#handleRefresh" text="Refresh" />
              </items>
            </ToolBar>
         </children>
      </VBox>
   </top>
</BorderPane>

RootLayoutController.java

public class RootLayoutController {

    private MainApp mainApp;
    
    @FXML
    private Tab tabKurse;
    @FXML
    private Tab tabDozenten;

    private KursOverviewController kursOverviewController;
    private DozentOverviewController dozentOverviewController;

    /**
     * Initializes the controller class.
     */
    public void initialize() {
        // load tab layouts + controllers
        kursOverviewController = loadTab(tabKurse, "KursOverview");
        dozentOverviewController = loadTab(tabDozenten, "DozentOverview");
    }

    /**
     * Loads the contents of the tab <tt>tab</tt>.
     * @see https://gist.github.com/nilsding/533e528fa9f2dac87b6c992158a8f45b
     * @author Georg G. <[email protected]>
     * @param tab reference to a JavaFX Tab
     * @param name Name of the FXML file without the extension.
     */
    private <T> T loadTab(Tab tab, String name) {
        try {
            FXMLLoader loader = new FXMLLoader();
            loader.setLocation(RootLayoutController.class.getResource(name + ".fxml"));
            SplitPane content = (SplitPane) loader.load();
            T controller = loader.getController();

            tab.setContent(content);

            return controller;
        } catch (IOException ex) {
            System.out.println("error while loading tab " + name);
            ex.printStackTrace();
            return null;
        }
    }
    
    public void setMainApp(MainApp mainApp) {
        this.mainApp = mainApp;
        // don't forget to set the mainapp in the underlying controllers as well if you need it there
        if (kursOverviewController != null) {
            kursOverviewController.setMainApp(mainApp);
        }
        if (dozentOverviewController != null) {
            dozentOverviewController.setMainApp(mainApp);
        }
    }
}