Null Reference exception when trying to extend the editor.

I’m just trying to make a window pop up from the Menu when clicking ‘Open’ but I get a null reference exception saying: Object reference not set to an instance of an object.

The code builds fine:

using UnityEngine;
using System.Collections;
using UnityEditor;
 

public class FFToolKit : MonoBehaviour {
   
	// Use this for initialization


    public string[] toolbarStrings = new string[] { "File", "View", "Edit" };
    public int toolbarInt = 0;
	void Start () {
	
	}
	
	// Update is called once per frame
	void Update () {
       
	}
    [MenuItem("FFToolKit/Open")]
    public static void ShowWindow()
    {
        EditorWindow.GetWindow(typeof(FFToolKit));
    }

    void OnGui()
    {
       toolbarInt = GUI.Toolbar(new Rect(5,5, 100, 20),toolbarInt, toolbarStrings);
    }
}

Any ideas as to where I’ve gone wrong?

You should derive/inherit from EditorWindow, not monobehaviour:

 using UnityEngine;
 using System.Collections;
 using UnityEditor;
  
 public class FFToolKit : EditorWindow 
 {
     public string[] toolbarStrings = new string[] { "File", "View", "Edit" };
     public int toolbarInt = 0;

     [MenuItem("FFToolKit/Open")]
     public static void ShowWindow()
     {
         EditorWindow.GetWindow(typeof(FFToolKit));
     }
 
     void OnGui()
     {
        toolbarInt = GUI.Toolbar(new Rect(5,5, 100, 20),toolbarInt, toolbarStrings);
     }
 }

Please see the Editor Window topic in the manual.