NullReferenceException from GetComponent on Instantiated object

[shinej777]

Hi,

A. Problem:
I’m writing a n-body gravity simulation and I’m currently getting the following NullReferenceException errors while trying to get a reference to an instantiated gameobject via GetComponent.

1. NullReferenceException: Object reference not set to an instance of an object Init.Awake () (at Assets/Scripts/Init.cs:12)

2. NullReferenceException: Object reference not set to an instance of an object GravityManager.FixedUpdate () (at Assets/Scripts/GravityManager.cs:53)

Any help would be greatly appreciated!

B. Expected behavior
Init.cs should:

1. Instantiate a GravityManager instance
2. Instantiate Sphere “sphere1” (line 12), which should be added to the gravity manager’s Sphere list “bodies”. <This is where the NullReferenceException is occurring>

The gravity manager instance should update the positions of each sphere in “bodies” each frame via FixedUpdate(). Currently, the manager only implements SHM about the origin using a 4th order symplectic integrator (Symplectic integrator - Wikipedia).

C. Code
(also at: orbits/Assets/Scripts at gm · Justin08784/orbits · GitHub)
Init.cs:

using UnityEngine;

public class Init : MonoBehaviour
{
    public GameObject Sphere_Prefab;
    public GameObject Gravity_Manager_Prefab;

    void Awake()
    {
        Instantiate(Gravity_Manager_Prefab, Vector3.zero, Quaternion.identity);
        GameObject sphereObject1 = Instantiate(Sphere_Prefab, new Vector3(2, 0, 0), Quaternion.identity);
        Sphere sphere1 = sphereObject1.GetComponent<Sphere>();
        // Sphere sphere2 = Instantiate(Sphere_Prefab, new Vector3(-2, 0, 0), Quaternion.identity).GetComponent<Sphere>();

        GravityManager.register_body(sphere1);
        // GravityManager.register_body(sphere2);
    }
}

GravityManager.cs:

using System;
using System.Collections.Generic;
using UnityEngine;

public class GravityManager : MonoBehaviour
{
    private static GravityManager Instance; // there should be only 1 manager
    private static List<Sphere> bodies;

    private readonly double c = Math.Pow(2.0, 1.0/3.0);

    /* integrator coeffs */
    double[] cs; // displacement update coeffs
    double[] ds; // velocity update coeffs

    private double dt = 0.01;

    private int upd_cnt = 0;

    private void Awake()
    {
        if (Instance != null) {
            // manager already init
            Destroy(this); 
            return;
        }

        Instance = this;
        DontDestroyOnLoad(Instance);

        cs = new double[4] {0.5, 0.5*(1.0-c), 0.5*(1.0-c), 0.5};
        ds = new double[4] {0.0,         1.0,          -c, 1.0};

        double divisor = 2.0 - c;
        for (int i = 0; i < cs.GetLength(0); i++) {
            cs[i] /= divisor;
            ds[i] /= divisor;
        }
    }

    public static void register_body(Sphere sphere)
    {
        bodies.Add(sphere);
    }
    double compute_accel(double x)
    {
        double k_by_m = 1; // k / m
        return -k_by_m * x;
    }

    void FixedUpdate()
    {
        Sphere curr_body = bodies[0];
        for (int i = 0; i < cs.GetLength(0); i++) {
            double a = compute_accel(curr_body.r.x);
            curr_body.v.x += (float) (ds[i] * a * dt);
            curr_body.r.x += (float) (cs[i] * curr_body.v.x * dt);
        }
        upd_cnt += 1;
    }
}

Sphere.cs:

public class Sphere : MonoBehaviour
{
    // Start is called before the first frame update
    public Vector3 v;
    public Vector3 r;

    void Start()
    {
        v = Vector3.zero;
        r = transform.position;
    }

    void FixedUpdate()
    {
        transform.position = r;
    }
}

[apkdev]

Hi, I think your code works correctly assuming that prefab references are hooked up correctly.

Note that register_body throws an exception because the bodies list is never created. (Also, I wouldn’t make that list static.)

As a side note, your repository doesn’t contain all the files necessary to open the project correctly. I recommend that you use .gitignore to set up which files you want to add to the repo. Here’s a template: gitignore/Unity.gitignore at main · github/gitignore · GitHub

[mlodinov]

As far as I can tell your prefab does not contain component “Sphere”. Maybe you can show your Inspector window when you select your prefab in project so I can see all components this prefab has?

[mlodinov]

Also quick advice, you can rewrite you code like this

public class Init : MonoBehaviour
{
    public Sphere Sphere_Prefab;

void Awake()
    {
        Sphere sphereObject1 = Instantiate(Sphere_Prefab, new Vector3(2, 0, 0), Quaternion.identity);
...

This way you can pass only prefabs that have Sphere component, and you will get Sphere component from instantiated object right away without using GetComponent

[shinej777]

I’ve pushed the rest of the project using the provided .gitignore (@apkdev).

Here is a screenshot of my “Sphere” prefab, (@mlodinov):

[mlodinov]

hey @shinej777 I checked your project and it seems like the problem is here

public class GravityManager : MonoBehaviour
{
	private static List<Sphere> bodies;

I changed it to this

public class GravityManager : MonoBehaviour
{
	private static List<Sphere> bodies = new(); 

and I don’t have any errors right now