Menu.cs

using UnityEngine;

public abstract class Menu<T> : Menu where T : Menu<T>
{
	public static T Instance { get; private set; }

	protected virtual void Awake()
	{
		Instance = (T)this; 
	}

	protected virtual void OnDestroy()
	{
		Instance = null;
	}

	protected static void Open() 
	{
		if (Instance == null)
		MenuManager.Instance.CreateInstance<T>();
		else
		Instance.gameObject.SetActive(true);
		
		MenuManager.Instance.OpenMenu(Instance, Instance.animInTime); 
	}

	protected static void Close()
	{
		if (Instance == null)
		{
			Debug.LogErrorFormat("Trying to close menu {0} but Instance is null", typeof(T));
			return;
		}

		MenuManager.Instance.CloseMenu(Instance, Instance.animOutTime); 
	}

	public override void OnBackPressed()
	{
		Close();
	}
}

public abstract class Menu : MonoBehaviour
{
	[Tooltip("Destroy the Game Object when menu is closed (reduces memory usage)")]
	public bool DestroyWhenClosed = true;

	[Tooltip("Disable menus that are under this one in the stack")]
	public bool DisableMenusUnderneath = true; 

	[Tooltip("Time to animate this menu in")]
	public float animInTime = 0f;

	[Tooltip("Time to animate this menu out")]
	public float animOutTime = 0f;

	public abstract void OnBackPressed();
}