NullReferenceException: Object reference not set to an instance of an object PlacingParts.Update () (at Assets/Scripts/BuildingRocket/PlacingParts.cs:46)

I am getting an error where I call the function SnapTry. I think it has something to do with my array that I have. Can anyone help me find out the problem? If you need me to tell you about what the script does than I can and maybe I can find a better way to achieve what I'm trying to if this won't work. Here's the script:

 using UnityEngine;
 using System.Collections;
 
 public class PlacingParts : MonoBehaviour 
 {
     public static GameObject[] placedParts;
     private int i = 0;
     private Transform currentPart;
     public Transform centerPoint;
 
     private int partLayerMask = 1 << 9;
     private int snapPointLayer = 1 << 8;
 
     private bool hasPlaced;
     public float snapPointDistance = 0.75f;
 
     void Start () 
     {
         hasPlaced = false;
     }
     void Update ()
     {
         //HOW TO PLACE THE ITEM
         if (currentPart != null && hasPlaced == false)
         {
             //GIVES ME THE ABILITY TO MOVE THE PART/OBJECT THAT WAS JUST INSTANTIATED WITH THE PLACEITEM FUNCTION
             Vector3 m =  Camera.main.ScreenToWorldPoint (new Vector3(Input.mousePosition.x, Input.mousePosition.y, 12.9f +( MouseOrbitImproved.distance - 13.5f)));
             currentPart.position = new Vector3(m.x,m.y,m.z);
 
             //SET THE ITEM TO HAS PLACED SO THAT IT STOPS MOVING
             if(Input.GetMouseButtonDown (0))
             {
                     hasPlaced = true;
                     placedParts[i + 1] = currentPart.gameObject;
             }
         
         }
         //HOW TO PICK THE OBJECT BACK UP AFTER IT HAS BEEN PLACED
         Ray ray = Camera.main.ScreenPointToRay (Input.mousePosition);
         RaycastHit hit;
         
         if(Physics.Raycast (ray, out hit, Mathf.Infinity, partLayerMask))
         {
         
         } 
         SnapTry (currentPart, placedParts [i].transform);
     }
         void SnapTry(Transform obj1, Transform obj2) {
             //WHEN CALLING THE SCRIPT OBJ1 MUST BE THE CURRENTPART
             Transform[] transforms1 = obj1.GetComponentsInChildren<Transform>();
             //OBJ2 HAS TO BE AN ARRAY FILLED WITH ALL THE PLACED OBJECTS TRANSFORMS
             Transform[] transforms2 = obj2.GetComponentsInChildren<Transform>();
 
             foreach(Transform tr1 in transforms1) {
                 foreach(Transform tr2 in transforms2) {
                     
                     if (tr1.tag == "SnapPoint" && tr2.tag == "SnapPoint") {
                         if ((tr1.position - tr2.position).sqrMagnitude <= snapPointDistance) {
                             Debug.Log ("You could snap an object here if the script was finsihed");
                         }
                     }
                 }
             }
         }
 
     public void PlaceItem(GameObject b)
     {
         hasPlaced = false;
         currentPart = (((GameObject)Instantiate(b)).transform);
     }
 }