Method reference dropping from OnClick()

Problem:
A method referenced by one of my buttons randomly comes up as “Missing”:

51166-untitled-picture.png

Details:
The referenced method is not static, although that shouldn’t be an issue in 5. Here’s the referenced script:

using UnityEngine;
using System.Collections;

public class MenuManager : MonoBehaviour
{
	// …
	//Referenced method
	public void ShowMenu(Menu menu)
	{
		if(menu.IsPopup == false)
		{
			//Closes the current menu
			if(CurrentMenu != null)
			{
				CurrentMenu.IsOpen = false;
				CurrentMenu.IsActive = false;
			}
			
			//Sets new menu (target of function) to CurrentMenu, and opens current menu
			CurrentMenu = menu;
			CurrentMenu.IsOpen = true;
			CurrentMenu.IsActive = true;
		}
		else
		{
			if(CurrentMenu != null)
			{
				CurrentMenu.IsOpen = false;
				CurrentMenu.IsActive = true;
			}
			
			CurrentMenu = menu;
			CurrentMenu.IsOpen = true;
			CurrentMenu.IsActive = true;
			
		}
	}
}

The object that OnClick is referencing has the component script on it, and the method in the component shows up in the OnClick drop-down menu in editor:

51167-untitled-picture2.png

Attempted Solutions:
I’ll replace the missing reference by reselecting it from the drop-down menu. Then I save and close Unity, reopen it a few times or work on other scripts, and the method shows up as Missing again. I’ve tried recreating the button wholesale, renaming the script, moving the script to a difference object in the Hierarchy and referencing it there-- all to no avail.

Desperately seeking help. Thanks for looking.

I am currently looking into this as well. What you got so far seems interesting.

Since this is from 2015, maybe you found a solution?