about

i publish this as a second answer cause it too big for comment 8)

NullReferenceException is rather the most popular error in unity. it just says that you have somewhere null (read: you haven't an object or you have nothing), but trying to call something for this object. of course, cause you havn't object, you can't do something with it.

for example, i want to set some speed to Rigidbody component of some gameobject. simply i will do it so:

myGameObject.rigidbody.velocity = Vector3.forward * 10f;

but, it's not safe cause an object here can absent. it can absent cause i forget to attach it or it was but is destroyed now. so, to be sure i should check it first:

if (myGameObject != null)
{
    myGameObject.rigidbody.velocity = Vector3.forward * 10f;
}
else
{
    ...do something when no object to attach speed
}

now i will not call to rigidbody if i have no gameObject. but, i still can get NullReferenceException, cause myGameObject can be without rigidbody, and myGameObject.rigidbody will return null. in this case myGameObject.rigidbody.velocity will raise error. so:

if (myGameObject != null)
{
    if (myGameObject.rigidbody == null)
    {
        myGameObject.AddComponent<Rigidbody>();
    }
    //at this point i absolutely sure that myGameObject is exists (checked it)
    //and it has a rigidbody (it was on GameObject or just created if wasn't)
    //so, now my call to velocity is safety
    myGameObject.rigidbody.velocity = Vector3.forward * 10f;
}
else
{
    ...do something when no object to attach speed
}

debug

task: we have some line of code with NullReferenceException. what's wrong?

answer:

for example, our line of code is:

myGameObject.rigidbody.velocity = Vector3.forward * 10f;

let's try to find null. insert step-by-step check using debug.log like here:

Debug.Log(myGameObject);
Debug.Log(myGameObject.rigidbody);
Debug.Log(myGameObject.rigidbody.velocity);
myGameObject.rigidbody.velocity = Vector3.forward * 10f;

now, if we got null on first debug.log, it means we have no gameobject. go to code above and look why this happens

if we got some gameobject in first debug but null in second, it means our gameobject somewhy has no rigidbody

if second debug shows component, third debug will show speed 8) it can't be null cause rigidbody always has speed

error points you directly to line where error occurs:

myGUI.Start () (at Assets/Scripts/HUD Classes/myGUI.cs:112)

this is method Start(), that you'd not publish. look for line 112 in myGUI.cs

publish whole method Start() if need explained.