Script not returning value to other script

Hi, basically my UI script gets value of force from user and converts it to float (this much working correctly)

 using UnityEngine;
 using System.Collections;
 using UnityEngine.UI;
 public class miUI : MonoBehaviour {
 
     public Text forceText;
     public Move move;
     public static float force=0;
     public FirstPersonController fps;
 
 
     // Use this for initialization
     void Start () {
         forceText.text = "Current Force : 0.0 N ";
 
     }
     
     public void CharacterField(string InputFieldString)
     {
         forceText.text = InputFieldString ;
         force = float.Parse (InputFieldString);
         Debug.Log (force);
     }
     public float returnForce()
     {   Debug.Log (force);
         return force;
     }
 }

I've got a script on my projectile prefab where it tries to get return value from returnForce function from UI script.But, instantiating my prefab instead gives the error that Object reference not set to an instance of object. Here's the script on my prefab.

 using UnityEngine;
 using System.Collections;
 
 public class Move : MonoBehaviour {
 
     Rigidbody rb;
     public static float force;
     public miUI ui;
 
     void Start () { 
         force = ui.returnForce();
         rb= GetComponent<Rigidbody>();
         rb.AddForce (transform.forward * force);
         Debug.Log (force);
     }
 
     public void forche( float inputForce)
     {
         force = inputForce;
     }
 }
 

All help appreciated !!

Edit: Exception is being thrown on line force = ui.returnForce();