Lambda expression button.setOnAction is not working outside the Main class - javafx

Question

I'm trying to attach a lambda expression to a button to fire a method. I want that lambda outside the Main method of class and outside a method. I have imported javafx.event.ActionEvent not to mix up with java.awt.event.ActionEvent. I might not be explaining the problem in an understandable way but hopefully the code will help.
Note: I have tried to use lambda expression within a method, outside a method, just to see if it works but it doesn't work outside a.
Error message is "Can't resolve symbol'setOnAction' "
package sample;
import javafx.fxml.FXML;
import javafx.fxml.FXMLLoader;
import javafx.scene.Node;
import javafx.scene.Parent;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.control.Label;
import javafx.stage.Stage;
import javafx.event.ActionEvent;
public class DashboardController {
#FXML
private Label lblHidden;
#FXML
private Button btnReveal;
#FXML
void backToLogin(ActionEvent event) throws Exception{
Parent myRoot = FXMLLoader.load(getClass().getResource("login.fxml"));
Scene loginScence = new Scene(myRoot);
Stage myStage = (Stage)((Node)event.getSource()).getScene().getWindow();
myStage.setScene(loginScence);
myStage.setTitle("Login");
myStage.show();
//Lambda works though not need here - inside a method
btnReveal.setOnAction(e->{
lblHidden.setText("Lambda inside a method - works");
});
}
//Lambda doesn't work here - -- I want it to work here
btnReveal.setOnAction(e->{`enter code here`
lblHidden.setText("Lambda doesn't work here");
});
}