DropDown menu(add item from input field): Object reference not set to instance of an object C#

My goal is: Type into the input field → press the button “Enter” → have that added to the drop down list.

My question is → How do I get rid of this NullReferenceException Error?

And if at all possible, how can I make this work?

The Error I am getting is this.

NullReferenceException: Object reference not set to an instance of an object
DdMenu.addItem()(at Assets/Scripts/DdMenu.cs: 20)

Here are my two scripts.

First script : This is for adding the options into the dropdown menu

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;

public class DdMenu : MonoBehaviour {

	AddOption ao;
	public Text selectedName;
	public Dropdown drop;


	public void dropIndex(int varIndex){

		selectedName.text = ao.menuList[varIndex];
	}

	public void addItem(){

		drop.AddOptions (ao.get_holder());  //Error is here
	}


}

Second Script: This is for getting the text from the input field.

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;

public class AddOption : MonoBehaviour {
	public List<string> menuList;
	public Text test; 

	private string x;
	public void getText(){

		x = test.text;
		menuList.Insert (0, x);

	}

	public List<string> get_holder(){
		return menuList;
	}
}

Read the error. One of the objects you’re attempting to access data from is null.

On line 20

drop.AddOptions (ao.get_holder());

you have two objects you’re attempting to act upon: drop and ao.

You have to assign these objects either in code or in the editor before attempting to use them like this.