C# Online Compiler | .NET Fiddle

using System;
using System.Linq;
using System.Collections.Generic;
using System.Diagnostics;

public class Program
{
	public class GameObject{
		public string name;
	}
	
   public class SetupWeapon {

        public int poolSize;
        public GameObject prefab;

        public SetupWeapon(int size, GameObject goPrefab) {
            this.poolSize = size;
            this.prefab = goPrefab;
        }
    }
	
	
	    public int[] AttackIDs {
        get {
            var toArray = _weaponData.Select(a => a.Key).ToArray();
            return toArray;
        }
    }
    private int[] PoolSize {
        get {
            var toArray = _weaponData.Select(a => a.Value.poolSize).ToArray();
            return toArray;
        }
    }
    private GameObject[] Prefab {
        get {
            var toArray = _weaponData.Select(a => a.Value.prefab).ToArray();
            return toArray;
        }
    }
	
	public Dictionary<int, SetupWeapon> _weaponData = new Dictionary<int, SetupWeapon>();
    public void Start () {

        for (int i = 0; i < AttackIDs.Length; i++){
            Console.WriteLine(AttackIDs[i]); // works just fine
            Console.WriteLine(_weaponData[0].poolSize); // works just fine
            Console.WriteLine(_weaponData[0].prefab.name); // works just fine
            Console.WriteLine(PoolSize[0]); // NullReferenceException
            Console.WriteLine(Prefab[0].name); // NullReferenceException
        }
    }
	
	public static void Main()
	{
		var p = new Program();
		p._weaponData.Add(0, new SetupWeapon(0, new GameObject(){ name = "0"}));
		p._weaponData.Add(1, null);  // this will cause null exception
		p._weaponData.Add(2, new SetupWeapon(2, null)); // this will not

		p.Start();
		
	}
}
Cached Result
Hello World