JavaFX: java.lang.NullPointerException while trying to resize borderpane - javafx

Question

Good evening guys,
I am still at the very beginning of Java programming. My current target is to resize the top of a BorderPane during the launch of the initialization. For sure, it would be very beneficial to have a separate class that contains the static constants (like the value for the size).
Unfortunately, I'm always getting a NullPointerException. I gues it's something really simple that I don't see. I hope that you can help me.
Thanks a lot in advance!
package main;
import javafx.application.Application;
import javafx.stage.Stage;
import javafx.scene.Scene;
import javafx.scene.layout.BorderPane;
import javafx.scene.layout.Pane;
import javafx.scene.layout.Region;
import javafx.fxml.FXML;
import javafx.fxml.FXMLLoader;
public class Main extends Application {
#FXML
Pane main_top;
#Override
public void start(Stage primaryStage) {
try {
BorderPane root = (BorderPane)FXMLLoader.load(getClass().getResource("MainGui.fxml"));
Scene scene = new Scene(root,400,400);
scene.getStylesheets().add(getClass().getResource("application.css").toExternalForm());
primaryStage.setScene(scene);
primaryStage.show();
primaryStage.setMaximized(true);
BorderPane.layoutInArea(main_top, 0, 0, 200, GlobalProperties.MainGuiTopPaneHeight, 0, null, false, false, null, null, false);
} catch(Exception e) {
e.printStackTrace();
}
}
EDIT:
java.lang.NullPointerException
at javafx.scene.layout.Region.layoutInArea(Unknown Source)
at main.Main.start(Main.java:27)
at com.sun.javafx.application.LauncherImpl.lambda$launchApplication1$153(Unknown Source)
at com.sun.javafx.application.LauncherImpl$$Lambda$51/2117598489.run(Unknown Source)
at com.sun.javafx.application.PlatformImpl.lambda$runAndWait$166(Unknown Source)
at com.sun.javafx.application.PlatformImpl$$Lambda$44/1051754451.run(Unknown Source)
at com.sun.javafx.application.PlatformImpl.lambda$null$164(Unknown Source)
at com.sun.javafx.application.PlatformImpl$$Lambda$47/1401292544.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at com.sun.javafx.application.PlatformImpl.lambda$runLater$165(Unknown Source)
at com.sun.javafx.application.PlatformImpl$$Lambda$46/1775282465.run(Unknown Source)
at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(Unknown Source)
at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
at com.sun.glass.ui.win.WinApplication.lambda$null$141(Unknown Source)
at com.sun.glass.ui.win.WinApplication$$Lambda$37/1109371569.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)
EDIT2 (MainGui.fxml):
<?xml version="1.0" encoding="UTF-8"?>
<?import javafx.scene.control.*?>
<?import java.lang.*?>
<?import javafx.scene.layout.*?>
<?import javafx.scene.layout.BorderPane?>
<BorderPane xmlns="http://javafx.com/javafx/8" xmlns:fx="http://javafx.com/fxml/1" fx:controller="main.MainGuiController">
<top>
<Pane fx:id="main_top" prefHeight="200.0" prefWidth="200.0" style="-fx-background-color: #525260;" BorderPane.alignment="CENTER" />
</top>
<left>
<Pane fx:id="main_left" prefHeight="200.0" prefWidth="200.0" style="-fx-background-color: #525252;" BorderPane.alignment="CENTER" />
</left>
<center>
<Pane fx:id="main_center" prefHeight="200.0" prefWidth="200.0" BorderPane.alignment="CENTER" />
</center>
<right>
<Pane fx:id="main_right" prefHeight="200.0" prefWidth="200.0" style="-fx-background-color: #525252;" BorderPane.alignment="CENTER" />
</right>
<bottom>
<Pane fx:id="main_bottom" prefHeight="200.0" prefWidth="200.0" style="-fx-background-color: #525260;" BorderPane.alignment="CENTER" />
</bottom>
</BorderPane>

Answer 1

Finally after some days of trial and error, I found a solution. Now, after it's solved it looks pretty easy ;-)
All I had to do was adding the controller initialization interface in the controller.
package main;
import java.net.URL;
import java.util.ResourceBundle;
import javafx.fxml.FXML;
import javafx.fxml.Initializable;
import javafx.scene.control.Button;
import javafx.scene.layout.Pane;
public class MainGuiController implements Initializable{
//***** #author n3wton
//**** FXML declarations for MainGui.fxml
#FXML
private Pane main_top;
#FXML
private Button button1;
//***** Beginning of auto-initialized method
#Override
public void initialize(URL arg0, ResourceBundle arg1) {
button1.setPrefSize(1, 1);
main_top.setPrefSize(50, 10);
}
}