Hey everybody, I've been getting hounded by a nullreference error while using array.Length in a C# editor script

This is the editor script (With unnecessary script lines removed)

using UnityEngine;
using UnityEditor;
using System.Collections;
    [CustomEditor(typeof(SentryTurretAI))]
    public class SentryTurretAIEditor : Editor {
        int sections;
        bool[] showSentryPart;
        string[] aimFromObjects;

        public override void OnInspectorGUI () {
           EditorGUIUtility.LookLikeControls();
           SentryTurretAI sentryTurretAI = (SentryTurretAI) target as SentryTurretAI;
           sections = sentryTurretAI.sentryObjects.Length;
    }

And this is the actual script affected:

using UnityEngine;
using System.Collections;

[System.Serializable]
public class trackObject {
        //These are variables in 'sentryObjects'
    public Transform rotatingObject;
    public float trackSpeed;
    public Vector3 up;
    public bool local;
    public bool x;
    public bool y;
    public bool z;
}

    [System.Serializable]
    public class SentryTurretAI : MonoBehaviour {
        public trackObject[] sentryObjects;
        //Removed clutter
        }

The above posted is what I've narrowed down as the problem, I've been working at it for a while and have had no prevail! To the best of my knowledge, this should be working but it gives this error (ignore line reference):

NullReferenceException: Object reference not set to an instance of an object SentryTurretAIEditor.OnInspectorGUI () (at Assets/Editor/SentryTurretAIEditor.cs:14) UnityEditor.InspectorWindow.DrawEditors (Boolean isRepaintEvent, UnityEditor.Editor[] editors, Boolean eyeDropperDirty) UnityEditor.InspectorWindow.OnGUI () System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture)

Any help would be absolutely amazing!

Thanks,

-AJ