How can I instantiate a prefab in a panel like slots and detect when it's full and make a scroll option?

Question

I try to make an vertical inventory, like i have show in picture.

I want to be able to add slots in number of items available, and and the create a schroolbar to view all slots available. i will use switch to switch the sprites. I need to create an instance of the prefab of the slot for each item picked. I have try my self implement ip but it give me an 2 errors wen i try to instatiate de prefab. this is the errors:

1 Setting the parent of a transform which resides in a prefab is 
disabled to prevent data corruption.
UnityEngine.Transform:SetParent(Transform, Boolean)
additem:CreateA() (at Assets/inventory ultimate pt/additem.cs:49)
additem:Start() (at Assets/inventory ultimate pt/additem.cs:35)

the line in question is
the void createA prefabtoinstatiate.transform.SetParent(panel.transform, false);


2 NullReferenceException: Object reference not set to an instance of an 
object
additem.CreateA () (at Assets/inventory ultimate pt/additem.cs:50)
additem.Start () (at Assets/inventory ultimate pt/additem.cs:35)

the line in question is
title_text.text = title_text1.text;

this is my code so far.

using UnityEngine;
using System.Collections;
using UnityEngine.UI;

public class additem : MonoBehaviour {
public static int havepick = 0;
public Transform prefabtoinstatiate;
public GameObject panel;
public static Text title_text;
public static Text description_text;
public static Text amount_text;
public static Sprite image;
public static int amountx = 0;

public Text title_text1;
public Text description_text1;
public Text amount_text1;
public Sprite image_icon_small_healling;
public Sprite image_icon_medium_healling;
public Sprite image_icon_big_healling;
public Sprite image_icon_small_stamina;
public Sprite image_icon_medium_stamina;
public Sprite image_icon_big_stamina;
public Sprite image_icon_small_magic;
public Sprite image_icon_medium_magic;
public Sprite image_icon_big_magic;
public Sprite image_icon_small_defence;
public Sprite image_icon_medium_defence;
public Sprite image_icon_big_defence;

public int image_number_count;
// Use this for initialization
void Start () {

    CreateA ();

}

// Update is called once per frame
void Update () {
    description_text1.text = "Small healling Potion " +
        "Give You 10 Points Of Healling";
    amount_text1.text = "Amount: " + amountx;
    title_text1.text= "Small Healling Potion";

}
public void CreateA() {

    prefabtoinstatiate.transform.SetParent(panel.transform, false);
    title_text.text = title_text1.text;
    description_text.text= description_text1.text;
    amount_text.text =amount_text1.text;
}
public void changeimages()
{
    switch (image_number_count) 
    {
        case 0:
        GetComponent <Image>().sprite = image_icon_small_healling;
        image_number_count++;
        break;
        case 1:
        GetComponent <Image>().sprite = image_icon_medium_healling;
        image_number_count++;
        break;
        case 2:
        GetComponent <Image>().sprite = image_icon_big_healling;
        image_number_count++;
        break;
        case 3:
        GetComponent <Image>().sprite = image_icon_small_stamina;
        image_number_count++;
        break;
        case 4:
        GetComponent <Image>().sprite = image_icon_medium_stamina;
        image_number_count++;
        break;
        case 5:
        GetComponent <Image>().sprite = image_icon_big_stamina;
        image_number_count++;
        break;
        case 6:
        GetComponent <Image>().sprite = image_icon_small_magic;
        image_number_count++;
        break;
        case 7:
        GetComponent <Image>().sprite = image_icon_medium_magic;
        image_number_count++;
        break;
        case 8:
        GetComponent <Image>().sprite = image_icon_big_magic;
        image_number_count++;
        break;
        case 9:
        GetComponent <Image>().sprite = image_icon_small_defence;
        image_number_count++;
        break;
        case 10:
        GetComponent <Image>().sprite = image_icon_medium_defence;
        image_number_count++;
        break;
        case 11:
        GetComponent <Image>().sprite = image_icon_big_defence;
        image_number_count++;
        break;
    }
}
}