Loading new scene in JavaFX - 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
}
}