Object does not instantiate, causing an error while attempting to get a component.

Question

I am presently working on translating a VR dodgeball game to the Oculus Quest 2, which sees the player use a ball to deflect oncoming balls. In this game, as the "Start Game" button is pressed, a coroutine should start, spawning the dodgeballs in the throwers' hands. In the Vive/SteamVR version of the application, the menu cannot be accessed by the player in the headset, instead an administrator manages the menu from the computer. The coroutine is shown below:

 public void spawnBall() {
             ball = Instantiate(ballPrefab, ballSpawnLocation.position, Quaternion.identity);
             ball.transform.localScale = new Vector3(EnemyController.ballSize, EnemyController.ballSize, EnemyController.ballSize);
             ball.transform.parent = ballSpawnLocation;
             ball.transform.rotation = Quaternion.LookRotation(-Vector3.forward);
 
             if (listener) {
                 GameObject.Find("StartGameButton").GetComponent<Button>().onClick.RemoveAllListeners();
                 listener = false;
             }
         }

I have recreated the menu so that the player inside the headset can access it and start the game. However, when the start button is pressed by the player, the coroutine does not run, and no balls spawn inside the throwers' hands. This results in a null reference exception, as the script controlling the throwers attempts to reference the Rigidbody component attached to the ball prefab.

 Rigidbody ballRigid = ball.GetComponent<Rigidbody>();

The ball never spawns, which also causes the throwers' animation to activate, however it does not reset into the idle animation. I am currently attempting to have the coroutine run again as the Start Game button is pressed, however I am unsure how to approach it.

Relevant photos are attached to this link: https://drive.google.com/drive/folders/13hM3iZpskkl-dk72PT0IPac29lnswRCB?usp=sharing

Answer 1

Answer by wideeyenow_unity · Jun 24, 2022 at 12:58 PM

I had an issue quite similar, the problem was my declaration of the object to be referenced. My exact issue was:

         body = transform.Find("Body").gameObject;
 
         shoulderL = transform.Find("Shoulder L").gameObject;
 
         shoulderR = transform.Find("Shoulder R").gameObject;
 
         armL = transform.Find("Shoulder L/Arm L").gameObject;
 
         armR = transform.Find("Shoulder R/Arm R").gameObject;

I didn't know that a "/" needed to be placed for children within children, as armL was just Find("Arm L") before. My null reference was only on Arm L(and a GetComponent of it), and nothing worked after it. No other errors, even though the other arm would have had the same issue. What is your error exactly, and where?

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jeisinn · Jun 24, 2022 at 01:20 PM 0

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My issue is specifically with the Rigidbody line, as it attempts to reference the Rigidbody on an object that was never instantiated. This also affects the throwers, since they no longer have a ball to throw, and it causes their throwing animation to run without returning to their idle animation on completion.

wideeyenow_unity jeisinn · Jun 24, 2022 at 02:29 PM 0

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You do not show the implementation of the rigidbody code, so I'm not fully sure on how that starts checking things. But I know a lot of people always refer to using Object Pooling, having all the objects needed spawn up during load, but if they are not used to be set to Inactive, and reside at (0,0,0) or wherever not important. And made active, and moved to proper position when they are needed. Which Object pooling also states to not destroy the object, simply make it inactive again and move to an unused location. And if you don't want to do Object Pooling, make sure the check for the rigidbody isn't called until the ball is instantiated. Which may, depending on your code setup, also require you to set that component/object to be inactive as well. And if you're using a parent class, the parent class will need to be the one setting active, or inactive, as the child object itself once Inactive will not read the code to turn itself on

jeisinn wideeyenow_unity · Jun 27, 2022 at 12:51 PM 0

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This is the function that gets the ball rigidbody, not including equations for ball trajectory and velocity:

  void ballLaunch() {
             Rigidbody ballRigid = ball.GetComponent<Rigidbody>();
 
             // Throw preparation
             ball.transform.parent = null;    // Removes parent
             ballRigid.useGravity = true;    // Turns on ball gravity
             ballRigid.constraints = RigidbodyConstraints.None;      // Removes positional constraints   
 
             ballPrefab.GetComponent<OnBallCollision>().StartTimeOut();        // Begins the timeout for the ball (if it hits nothing, count as a fail)
 
             GetComponent<Animator>().SetBool("IsThrowing", false);      // Stop the throwing animation

The rigidbody is referenced once the ball is to be launched, but since the ball is never instantiated, it returns the null reference exception.

Answer 2

Answer by jeisinn · Jul 05, 2022 at 12:30 PM

I figured out that this was the result of me having more than one UI object with the startGame function. It seems like the game becomes confused when more than one button starts the game, and does not instantiate the balls as expected.

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Object does not instantiate, causing an error while attempting to get a component.

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