Error after Build - NullReferenceException: Object reference not set to an instance of an object

For simplified context, I'm making a prototype 2D platformer with a main menu screen, and 3 levels with transitions in between them. The errors are: (1) there are no transition events, (2) cannot move to next levels from Player win-condition-collision detection. The relevant scripts are:

1. The UI Manager handles a screen transition animation event (fade to black, fade to white), and other UI elements such as a win screen.

2. The Game Manager handles building next level from player collision detection, restarting levels and sending inputs to UI Manager to do the transition.

3. The PlayerController handles inputs for movement, jump, and trigger collision with the win condition, which sends an input to the game manager.

These are what I believe are considering the chain reaction of errors (but I might be wrong) - however I do not fully understand what I am doing wrong, as it appears perfect in editor mode, but not in build.

These are the errors found in the player log.

And the relevant errors(?) in the code: PlayerController:

 public void OnTriggerEnter2D(Collider2D collision)
     {
         if (isCollidingwithWinCondition) return;
         if (collision.CompareTag("VictoryCondition"))
         {                       
             isCollidingwithWinCondition = true;            
             GameManager.instance.LoadNextLevel();
             StartCoroutine(Reset());
         }
     }
 IEnumerator Reset()
     {
         yield return new WaitForSeconds(1);
         isCollidingwithWinCondition = false;
     }

GameManager:

 public void LoadNextLevel()
     {
         //Get Active Scene
         thisscene = SceneManager.GetActiveScene();
 
         if (thisscene.buildIndex == SceneManager.sceneCountInBuildSettings - 1)
         {
             //Finish Game if win all Level
             WinGame();
             return;
         }
         else
         {
             //Waiting for Transition            
             UIManager.instance.StartTransition();
             StartCoroutine(Wait());
         }
     }
  IEnumerator Wait()
     {
         float TransitionTime = UIManager.instance.TransitionTime;
         yield return new WaitForSeconds(TransitionTime);
 
         //Load NextLevel
         thisscene = SceneManager.GetActiveScene();
         SceneManager.LoadScene(thisscene.buildIndex + 1);
     }

UIManager:

     void Start()
     {
         #region Singleton
         if (instance == null)
             instance = this;
 
         else if (instance != this)
             Destroy(gameObject);
 
         DontDestroyOnLoad(gameObject);
         #endregion
 
         Scene0 = SceneManager.GetSceneByBuildIndex(0).buildIndex;
         TransitionController = GetComponent<Animator>();
     }
 public void StartTransition()
         {
             TransitionController.SetTrigger("Start");
         }