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;
     }
 }