Null exception for public variable

Hello! I have been trying to get one script to read input from another. I have this script so far:

The sender script:

 using UnityEngine;
 using System.Collections;
 
 public class Test: MonoBehaviour 
 
 {
     public jumpAfterTest _jumpAfterTest;
 
     void OnMouseDown() 
     {
         Debug.Log ("Mouse Down");
 
         _jumpAfterTest.JAT ();
     }
 
 void OnMouseUp()
     {
         Debug.Log ("Mouse up");
 
     }
     
 }

And the receiver script:

 using UnityEngine;
 using System.Collections;
 
 public class jumpAfterTest : MonoBehaviour 
 {   
     public void JAT()
     {
         Debug.Log("Me is JAT");
     }
 }

The error I am getting is:

NullReferenceException: Object reference not set to an instance of an object Test.OnMouseDown () (at Assets/my-scripts/Test.cs:13) UnityEngine.SendMouseEvents:DoSendMouseEvents(Int32, Int32)

I am guessing that this error has to do with line 13 of test script: _jumpAfterTest.JAT ();

How would I fix this? Thanks for the help! I am in mobile, by the way.