Loading new scene in JavaFX

Question

I am attempting to switch scenes from a login screen to the main screen of my program but whenever I try to switch scenes after clicking login, I get the following error.

"Exception in thread "JavaFX Application Thread" java.lang.RuntimeException: java.lang.reflect.InvocationTargetException"

I've messed around with my code and try to change some things to get different results but no dice. This is my first time doing a GUI so any help would be appreciated.

package pwmanager;

import java.io.IOException;
import java.net.URL;
import java.util.ResourceBundle;
import javafx.event.ActionEvent;
import javafx.event.EventType;
import javafx.fxml.FXML;
import javafx.fxml.FXMLLoader;
import javafx.fxml.Initializable;
import javafx.scene.Parent;
import javafx.scene.Scene; 
import javafx.scene.control.*;
import javafx.scene.layout.AnchorPane;
import javafx.scene.layout.GridPane;
import javafx.stage.Stage;


/**
*
* @author 176878
*/
public class FXMLDocumentController implements Initializable {

@FXML
private Button loginButton;


@FXML
Stage prevStage;
Stage currentStage;
public void setPrevStage(Stage stage){
    this.prevStage = stage;

   }

@FXML
public void getPrevStage(Stage stage){
    this.currentStage = prevStage;
}

@FXML
public void loginButtonAction(ActionEvent event) throws IOException {
   System.out.println("You clicked me, logging in!");
    setPrevStage(prevStage);

    Stage stage = new Stage();

    try{ 
        FXMLLoader loader = new FXMLLoader(getClass().getResource("MainScreen.fxml"));

    GridPane mainScreen = (GridPane) loader.load();
    Scene scene = new Scene(mainScreen);
    stage.setScene(scene);
    stage.setTitle("Password Manager");
    stage.show();
    prevStage.hide();
      }

    catch(IOException e){
        System.out.println("Did not load right");
    }

         }

@Override
public void initialize(URL url, ResourceBundle rb) {
    // TODO
}    

}