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

some self education would be appreciated