Prefab in inspector not getting assigned

I'm trying to make an infinite runner game where enemy prefabs randomly spawn and move down the line, however unity keeps freezing and cras$$anonymous$$ng when I run the scene. T$$anonymous$$s is my controller.cs that will control the enemy creation and movement.

 public enum GameState { playing, gameover };
 
 public class Controller : MonoBehaviour {
 
     public ArrayList enemies;
     public static GameState gameState;
     public float targetsSpawnedUpTo = 0.0f;
     public Transform enemyPrefab ;//t$$anonymous$$s is giving me problems later on
     private Transform playerTrans;
     public float enemySpeed = 20.0f;
     
     void Awake()
     {
         enemies = new ArrayList();
         SpawnTargets(25.0f);
         StartGame();
     }
     
     void FixedUpdate()
     {
         
     }
     
     void StartGame()
     {
         Time.timeScale = 1.0f;
         gameState = GameState.playing;
     }
     
     void GameOver()
     {
         Time.timeScale = 0.0f;//pause
         gameState = GameState.gameover;
         //check score
     }
     
     void SpawnTargets(float upTo)
     {
         float spawnHeight = targetsSpawnedUpTo;    
         w$$anonymous$$le(spawnHeight <= upTo)
         {
             float x = Random.Range(-3.0f, 3.0f);
             Vector3 pos = new Vector3(x, 1f, spawnHeight);
             
             Transform badGuy = (Transform)Instantiate(enemyPrefab, pos, Quaternion.identity);//t$$anonymous$$s makes unity freeze and crash
             enemies.Add(badGuy);
             
             spawnHeight += Random.Range(-12.0f, -20.0f);
         }
         targetsSpawnedUpTo = upTo;
     }
 }

I've dragged my enemyPrefab into the inspector where it says "Enemy Prefab: None(Transform)" but then when I run the scene it freezes and gives me a "Object reference not set to an instance on the line where I instantiate the enemyPrefab.