Проблема с кодом

Код не находит dropdown, хотя он создан и находится в сцене. Вот код:
using UnityEngine;
using UnityEngine.UI;

public class Resolution : MonoBehaviour
{
public Dropdown DropDown;

// Start is called once before the first execution of Update after the MonoBehaviour is created
void Start()
{

}

// Update is called once per frame
void Update()
{

}

public void DropD()
{
if (DropDown.value == 0){
Screen.SetResolution(2560,1440,true);
}
if (DropDown.value == 1){
Screen.SetResolution(1920,1080,true);
}
if (DropDown.value == 2){
Screen.SetResolution(1680,1050,true);
}
if (DropDown.value == 3){
Screen.SetResolution(1600,1200,true);
}
if (DropDown.value == 4){
Screen.SetResolution(1440,900,true);
}
if (DropDown.value == 5){
Screen.SetResolution(1400,1050,true);
}
if (DropDown.value == 6){
Screen.SetResolution(1280,1024,true);
}
if (DropDown.value == 7){
Screen.SetResolution(1152,864,true);
}
if (DropDown.value == 8){
Screen.SetResolution(1024,768,true);
}
if (DropDown.value == 9){
Screen.SetResolution(800,600,true);
}
if (DropDown.value == 10){
Screen.SetResolution(640,480,true);
}
}
}