Order of object instantiation scripting issues

If the title needs clarification, the trouble I'm having is with the order by which objects are created.

For example, I have an "HUD" script, where the enemies created by instantiation will have the working health bar HUD, but objects of the same prefab placed in the scene don't have one. I get the error "internal_create can only be called from the main thread" and "null reference exception, unityEngine.texture.get_width()"

Another example is that when I place an object in the scene, my "find target" script will work, and I can get a component from that objects script. However, when an object is created through instantiation, I can't access that variable. I get the error "Object reference not set to an instance of an object"

Are there certain commands I need to use to reference a prefab object?

Here is the HUD script:

 using UnityEngine;
 using System.Collections;
 
 public class behavior : MonoBehaviour {
     
     public Transform target;
     public float maxHp;
     public float curHp;
     
     GUIStyle style = new GUIStyle();
     Texture2D texture = new Texture2D (128, 128);
     public Vector3 hpPos;
     
     void Start()
     {    
         curHp = maxHp;
     }
     
     void Update () 
     {
         hpPos = Camera.main.WorldToScreenPoint (transform.position);
         hpPos.y = Screen.height - hpPos.y;
         
         hpPos.z = -1;
         
         target = GameObject.FindGameObjectWithTag ("myPlayer").transform;
     }
     
     void OnGUI()
     {
         for (int i = 0; i < texture.width; i++) 
         {
             for(int j = 0; j < texture.height; j++)
             {
                 Color color = new Color(255, 0, 0);
                 texture.SetPixel(i, j, color);
             }
         }
         
         texture.Apply ();
         style.normal.background = texture;
         GUI.Box (new Rect(hpPos.x - 15, hpPos.y + 45, curHp, 7), new GUIContent(""), style);
     }
     
 
 }

and here is my "find target" script:

 using UnityEngine;
 using System.Collections;
 
 public class combat : MonoBehaviour {
     
     public Transform target;
 
     private behavior curHp;
 
 
     void Start()
     {
         target = GameObject.FindGameObjectWithTag ("enemy").transform;
         curHp = target.GetComponent<behavior>();
 
         Debug.Log (target);
         Debug.Log ("Enemies HP = " + curHp.curHp);
                 
     }
 }