Unable to assign array data on scriptable object

I am trying to create a scriptable object via editor menu to store some data. When I try to assign ordinary fields I don't get any problems but if I try to assign an array I get a NullReferenceException.

The following code works:

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 using System.Linq;
 
 [System.Serializable]
 public class EachLevel
 {
     public uint mLevelID;
     public uint mSceneID;
 };
 
 
 [System.Serializable]
 public class LevelData : ScriptableObject
 {
     public EachLevel lvl;
 
     public void Init ( uint id1, uint id2 )
     {
         lvl.mLevelID = id1;
         lvl.mSceneID = id2;
     }
 }

But this one does not:

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 using System.Linq;
 
 [System.Serializable]
 public class EachLevel
 {
     public uint mLevelID;
     public uint mSceneID;
 };
 
 
 [System.Serializable]
 public class LevelData : ScriptableObject
 {
     public EachLevel[] lvl;
 
     public void Init ( uint id1, uint id2 )
     {
         lvl = new EachLevel[2];
 
         lvl[0].mLevelID = id1;
         lvl[0].mSceneID = id2;
 
         lvl[1].mLevelID = id2;
         lvl[1].mSceneID = id1;
     }
 }

Any idea why?