ArgumentException: The prefab you want to instantiate is null.

I am rather new to Unity and C#, and need help. I have looked around on the forums for people with similar problems but have yet to find anything that helps.

There are two game objects that need to be instantiated a wolf-object and a rabbit-object. I have scripted them in two different ways which seem the same to me. Why is does instantiating the wolf-object throw the above error and how can I correct this? In both cases the prefab has been assigned to their relevant scripts.

 using UnityEngine;
 using System;
 using System.Collections;
 using System.IO;
 using System.Linq;
 
 public class SpeciesCreation : MonoBehaviour {     
 
     public GameObject RabbitObj;
     
     int RabbitInit=30;
     
     WolfProps p=new WolfProps();
     
     void Start () 
     {    
         p.WolfCreate();
         
         for(int i=1;i<=RabbitInit;i++)
         {
             var RabbitObject = GameObject.Instantiate(RabbitObj, new Vector3((float)(UnityEngine.Random.value-0.5)*90f+50f,100.25f,(float)(UnityEngine.Random.value-0.5)*90f+50f),Quaternion.identity);
     }            
     }
     void Update ()
     {
         EquilibriumMathEngine.EquilibriumCalc();
     }
 }


 
 using UnityEngine;
 using System.Collections;
 
 public class WolfProps : MonoBehaviour 
 {    
     public GameObject WolfObj;
 
     
     public int WolfCreate()
     {
         {    
             var WolfObject = GameObject.Instantiate(WolfObj, new Vector3((float)(UnityEngine.Random.value-0.5)*90f+50f,101.25f,(float)(UnityEngine.Random.value-0.5)*90f+50f),Quaternion.identity);
             return 1;
         }
     }    
 }

Thank you in advance for any help :)