Unity 5 change button text created with UnityScript - unity3d

Question

In my 2D project I create a canvas and a button in code. I would like to set the text of the button in code, but after numerous attempts I can't seem to do it.
My UnityScript code:
#pragma strict
var loginButton : UnityEngine.UI.Button;
function Start () {
var canvas = new GameObject ("canvas", Canvas);
var instance : UnityEngine.UI.Button = Instantiate(loginButton);
instance.GetComponent(UnityEngine.UI.Text).text = "login"; //Error below
instance.transform.position = Vector2(0,0);
instance.transform.SetParent(canvas.transform);
}
This provides an error
"NullReferenceException: Object reference not set to an instance of an object
GameLogicLogin.Start () (at Assets/GameLogicLogin.js:11)"
--------------------------------Edit---------------------------------------
The generated hierarchy looks like this: http://puu.sh/iMYe6/4f4a8f545c.png
On the left at the bottom is the prefab I link to the script.
The following line doesn't cause an error and seems to change the text, but the change doesn't show up in game nor does the original text assigned to the "Text"in the prefab.
instance.GetComponentInChildren(UnityEngine.UI.Text).text = "login";

Answer 1

If you would of built your button in the unity editor you could easily change its text in script like this:
var button = GameObject.Find("buttonObjectName").GetComponent<Button>();
button.GetComponentInChildren<Text>().text = "What ever you like";
I imagine you can easily adjust this to fit your needs without being spoon fed.

Answer 2

You are instantiating a UnityEngine.UI.Button, that doesn't have a component for UnityEngine.UI.Text hence the error.
Is there any reason why you are instantiating that object?
If you are linking in the editor a game object to loginButton, and I think you are, you should do something like this:
var comps = loginButton.gameObject.GetComponentsInChildren(UnityEngine.UI.Text);
comps[0].text = "login";