Unity script reference is null depend on which function use the reference - unity3d

Question

I've made a script that take some public object reference.
But depending on which function i call, one reference is fine but in the other function it's null, i don't understand why.
In the function onSave(), the reference to database is fine and i can access her function like add.
Then, when i call the function edit (which is supposed to retrieve an object into database and fill the form with data) the reference to database is null.
Error log (line 33 is this one: Step step = this.database.get(this.scrollList.selectedItem);:
> NullReferenceException: Object reference not set to an instance of an object
StepForm.edit () (at Assets/Prefabs/Scripts/StepForm.cs:33)
UnityEngine.Events.InvokableCall.Invoke (System.Object[] args) (at C:/buildslave/unity/build/Runtime/Export/UnityEvent.cs:110)
UnityEngine.Events.InvokableCallList.Invoke (System.Object[] parameters) (at C:/buildslave/unity/build/Runtime/Export/UnityEvent.cs:574)
UnityEngine.Events.UnityEventBase.Invoke (System.Object[] parameters) (at C:/buildslave/unity/build/Runtime/Export/UnityEvent.cs:716)
UnityEngine.Events.UnityEvent.Invoke () (at C:/buildslave/unity/build/Runtime/Export/UnityEvent_0.cs:53)
UnityEngine.UI.Button.Press () (at C:/buildslave/unity/build/Extensions/guisystem/UnityEngine.UI/UI/Core/Button.cs:35)
UnityEngine.UI.Button.OnPointerClick (UnityEngine.EventSystems.PointerEventData eventData) (at C:/buildslave/unity/build/Extensions/guisystem/UnityEngine.UI/UI/Core/Button.cs:44)
UnityEngine.EventSystems.ExecuteEvents.Execute (IPointerClickHandler handler, UnityEngine.EventSystems.BaseEventData eventData) (at C:/buildslave/unity/build/Extensions/guisystem/UnityEngine.UI/EventSystem/ExecuteEvents.cs:52)
UnityEngine.EventSystems.ExecuteEvents.Execute[IPointerClickHandler] (UnityEngine.GameObject target, UnityEngine.EventSystems.BaseEventData eventData, UnityEngine.EventSystems.EventFunction`1 functor) (at C:/buildslave/unity/build/Extensions/guisystem/UnityEngine.UI/EventSystem/ExecuteEvents.cs:269)
UnityEngine.EventSystems.EventSystem:Update()
here is my script:
using UnityEngine;
using UnityEngine.UI;
using System.Collections;
public class StepForm : MonoBehaviour {
public Database database;
public ScrollList scrollList;
public InputField title;
public InputField content;
private string uid;
// Use this for initialization
void Start () {
}
// Update is called once per frame
void Update () {
}
public void onSave(){
Step newStep = new Step (this.title.text, this.content.text);
this.database.addStep (newStep);
}
public void onCancel(){
Debug.Log ("onCancel");
}
public void onDelete(){
Debug.Log ("onDelete");
}
public void edit(){
Step step = this.database.get(this.scrollList.selectedItem);
if (step != null) {
this.title.text = step.Title;
this.content.text = step.Content;
}
}
}
The script reference source:
using UnityEngine;
using System.Collections;
using System.Collections.Generic;
public class Database : MonoBehaviour {
// Update is called once per frame
void Update () {
}
private List<Step> steps = new List<Step>();
// Use this for initialization
void Start () {
}
public List<Step> Steps {
get {
return this.steps;
}
}
public void addStep(Step newStep)
{
//if (!this.exist(newStep.Uid))
this.steps.Add(newStep);
}
public void deleteStep (Step newStep)
{
this.steps.Remove(newStep);
}
public void updateStep(string uid, Step newStep)
{
Step step = this.Steps.Find(o => o.Title == newStep.Title);
if (step != null) {
step.Title = newStep.Title;
step.Content = newStep.Content;
}
}
private bool exist(string uid)
{
if (this.Steps.Find (o => o.Uid == uid) != null)
return (true);
return (false);
}
public Step get(string uid)
{
Step step = this.Steps.Find (o => o.Uid == uid);
Debug.Log ("GET DATABASE");
Debug.Log (step.Title);
return (step);
}
public void debugContent(){
foreach (Step step in this.steps) {
Debug.Log ("### Debug Step ###");
Debug.Log(step.Uid);
Debug.Log(step.Title);
Debug.Log(step.Content);
}
}
public void clearDatabase(){
this.steps.Clear ();
Debug.Log ("Database cleared!");
}
}