NullReferenceException: Object reference not set to an instance of an object

I don't understand what's wrong.

 //SpeedController.cs
 using UnityEngine;
 using System.Collections;
 
 public class SpeedController : MonoBehaviour {
     public float speed;
     void Start(){
         speed=0.1f;
     }
     void Update(){
         Debug.Log (speed);
     }
 }
 

 //ClickController.cs
 using UnityEngine;
 using System.Collections;
 
 public class ClickController : MonoBehaviour {
     private GameObject[] cubes;
     public SpeedController _SpeedController;
     private float thisSpeed;
     void Start(){
         //thisSpeed = 0.1F;
         cubes=GameObject.FindGameObjectsWithTag("SquaresTag");
         InvokeRepeating ("Expand",1,0.03F);
         thisSpeed = 0.1F;
     }
     void Expand(){
         if(transform.position.z>0){
             transform.position -= new Vector3 (0,0,thisSpeed);
         }
     }
     void OnMouseDown(){
         foreach(GameObject hello in cubes){
             hello.transform.position+= new Vector3(0,0,10);
             _SpeedController.speed+=0.1f;
             thisSpeed=_SpeedController.speed;
         }
     }
 }

Can you help please? Thanks