GameManager.cs

using Godot;
using System;
using flappy_bird;

public partial class GameManager : Node2D
{
	private Button newGameButton;
	private CanvasLayer canvasLayer; // CanvasLayer for UI elements
	private static GameManager _instance;

	// public static GameManager Instance
	// {
	// 	get
	// 	{
	// 		return _instance;
	// 	}
	// }
	public static GameManager Instance => _instance;

	public override void _Ready()
	{
		if (GameManager.Instance == null)
			if (_instance == null)
			{
				_instance = this;
				SetProcess(true); //makes sure that the GameManager is not disposed 
			}
			else
			{
				QueueFree(); // If a diff GameManager exists, remove this one
				return;
			}

		canvasLayer = new CanvasLayer();  
		AddChild(canvasLayer);
		BirdSpeedTestWithReflection.TestBirdSpeed();
	}

	public void GameOver()
	{
		if (_instance == null || !IsInstanceValid(_instance))
		{
			GD.PrintErr("GameManager instance is invalid or disposed.");  // for debugging
			return;
		}

		GD.Print("Game Over"); //if body is player(from pipe class), print "Game Over"
		GetTree().Paused = true; // Pause the game

		newGameButton = new Button();
		newGameButton.Text = "New Game";
		newGameButton.AddThemeColorOverride("font_color", new Color(0, 1, 0)); // green text

		var viewportSize = GetViewportRect().Size; // Get the size of the visible area of game window
		var buttonSize = new Vector2(200, 50);
		newGameButton.Size = buttonSize;
		newGameButton.Position = (viewportSize - buttonSize) / 2; // Center the button on screen

		newGameButton.ProcessMode = ProcessModeEnum.Always; // Button always processed even if game is paused
		newGameButton.Show();

		newGameButton.Pressed +=
			OnNewGameButtonPressed; // Connect the button's pressed signal to the OnNewGameButtonPressed method
		// newGameButton.Connect("pressed", Callable.From(OnNewGameButtonPressed));  // Connect the button's pressed signal to the OnNewGameButtonPressed method

		canvasLayer.AddChild(newGameButton); // Add the button to the CanvasLayer
	}

	private void OnNewGameButtonPressed()
	{
		GD.Print("Pressed New Game Button"); //logging to console

		// Check if the button exists and is in the scene tree. If yes then remove it before reloading the scene
		if (newGameButton != null && newGameButton.IsInsideTree())
		{
			newGameButton.QueueFree(); // Remove the button from the scene tree
		}

		_instance = null; // Reset the instance before reloading the scene otherwise we will have no instances of GameManager and will get errors
		GetTree().Paused = false;
		GetTree().ReloadCurrentScene();
	}

	public override void _Process(double delta)
	{
	}
}