Can't use value from fxml in java class - javafx?

Question

So if i try to use a button from the fxml using the @FXML i get a nullpointer exception or nothing happens. I'm new with javafx so go easy on me, but no matter what i try it doesnt seem to work...

In the fxml file i have a button created in scenebuilder with fx:id="companies" - this all works fine and is definitely implemented properly.

In my java main file this is the code:

package sample;

import javafx.application.Application;
import javafx.fxml.FXML;
import javafx.fxml.FXMLLoader;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.image.Image;
import javafx.scene.image.ImageView;
import javafx.scene.layout.BorderPane;
import javafx.stage.Stage;

import java.io.File;


public class Main extends Application {

    @FXML private Button companies;

    @Override
    public void start(Stage primaryStage) throws Exception{

        companies.setOnAction(event -> {
           companies.setText("new value");
             });

        File file = new File("/users/Joss/Documents/continents.gif");

        Image image = new Image(file.toURI().toString());
        ImageView imageview = new ImageView(image);

        BorderPane pane = FXMLLoader.load(Main.class.getResource("sample.fxml"));
        //mainPane.setCenter(imageview);

        Scene scene = new Scene(pane, 1000, 800);
        primaryStage.setScene(scene);
        primaryStage.show();
    }


    public static void main(String[] args) {
        launch(args);
    }
}

and this is the error: Caused by: java.lang.NullPointerException at sample.Main.start(Main.java:24)