So I was following along by this tutorial, on how to make a drivable car in Unity. After I got to the point when you press the play button and hope for the best, I got an error, which was saying:
“NullReferenceException: Object reference not set to an instance of an object
PlayerCar_Script.Update () (at Assets/PlayerCar_Script.js:65)”
Since I’m new to the Unity and C#/JS scripting, there is probably some stupid mistake I am making. Here are the codes:
PlayerCar_Script.js
// These variables allow the script to power the wheels of the car.
var frontLeft : WheelCollider;
var frontRight : WheelCollider;
// These variables are for the gears, the array is the list of ratios. The script
// uses the defined gear ratios to determine how much torque to apply to the wheels.
private var GearRatio : float[];
var CurrentGear : int = 0;
// These variables are just for applying torque to the wheels and shifting gears.
// using the defined Max and Min Engine RPM, the script can determine what gear the
// car needs to be in.
var EngineTorque : float = 600.0;
var MaxEngineRPM : float = 3000.0;
var MinEngineRPM : float = 1000.0;
private var EngineRPM : float = 0.0;
function Start () {
// I usually alter the center of mass to make the car more stable. I'ts less likely to flip this way.
rigidbody.centerOfMass.y = -1.5;
}
function ShiftGears() {
// this funciton shifts the gears of the vehcile, it loops through all the gears, checking which will make
// the engine RPM fall within the desired range. The gear is then set to this "appropriate" value.
if ( EngineRPM >= MaxEngineRPM ) {
var AppropriateGear : int = CurrentGear;
for ( var i = 0; i < GearRatio.length; i ++ ) {
if ( frontLeft.rpm * GearRatio[i] < MaxEngineRPM ) {
AppropriateGear = i;
break;
}
}
CurrentGear = AppropriateGear;
}
if ( EngineRPM <= MinEngineRPM ) {
AppropriateGear = CurrentGear;
for ( var j = GearRatio.length-1; j >= 0; j -- ) {
if ( frontLeft.rpm * GearRatio[j] > MinEngineRPM ) {
AppropriateGear = j;
break;
}
}
CurrentGear = AppropriateGear;
}
}
function Update () {
// This is to limith the maximum speed of the car, adjusting the drag probably isn't the best way of doing it,
// but it's easy, and it doesn't interfere with the physics processing.
rigidbody.drag = rigidbody.velocity.magnitude / 250;
// Compute the engine RPM based on the average RPM of the two wheels, then call the shift gear function
EngineRPM = (frontLeft.rpm + frontRight.rpm)/2 * GearRatio[CurrentGear];
ShiftGears();
// finally, apply the values to the wheels. The torque applied is divided by the current gear, and
// multiplied by the user input variable.
frontLeft.motorTorque = EngineTorque / GearRatio[CurrentGear] * Input.GetAxis("Vertical");
frontRight.motorTorque = EngineTorque / GearRatio[CurrentGear] * Input.GetAxis("Vertical");
// the steer angle is an arbitrary value multiplied by the user input.
frontLeft.steerAngle = 10 * Input.GetAxis("Horizontal");
frontRight.steerAngle = 10 * Input.GetAxis("Horizontal");
}
WheelAlignment_Script.js
// Define the variables used in the script, the Corresponding collider is the wheel collider at the position of
// the visible wheel, the slip prefab is the prefab instantiated when the wheels slide, the rotation value is the
// value used to rotate the wheel around it's axel.
var CorrespondingCollider : WheelCollider;
var SlipPrefab : GameObject;
private var RotationValue : float = 0.0;
function Update () {
// define a hit point for the raycast collision
var hit : RaycastHit;
// Find the collider's center point, you need to do this because the center of the collider might not actually be
// the real position if the transform's off.
var ColliderCenterPoint : Vector3 = CorrespondingCollider.transform.TransformPoint( CorrespondingCollider.center );
// now cast a ray out from the wheel collider's center the distance of the suspension, if it hit something, then use the "hit"
// variable's data to find where the wheel hit, if it didn't, then se tthe wheel to be fully extended along the suspension.
if ( Physics.Raycast( ColliderCenterPoint, -CorrespondingCollider.transform.up, hit, CorrespondingCollider.suspensionDistance + CorrespondingCollider.radius ) ) {
transform.position = hit.point + (CorrespondingCollider.transform.up * CorrespondingCollider.radius);
}else{
transform.position = ColliderCenterPoint - (CorrespondingCollider.transform.up * CorrespondingCollider.suspensionDistance);
}
// now set the wheel rotation to the rotation of the collider combined with a new rotation value. This new value
// is the rotation around the axle, and the rotation from steering input.
transform.rotation = CorrespondingCollider.transform.rotation * Quaternion.Euler( RotationValue, CorrespondingCollider.steerAngle, 0 );
// increase the rotation value by the rotation speed (in degrees per second)
RotationValue += CorrespondingCollider.rpm * ( 360/60 ) * Time.deltaTime;
// define a wheelhit object, this stores all of the data from the wheel collider and will allow us to determine
// the slip of the tire.
var CorrespondingGroundHit : WheelHit;
CorrespondingCollider.GetGroundHit( CorrespondingGroundHit );
// if the slip of the tire is greater than 2.0, and the slip prefab exists, create an instance of it on the ground at
// a zero rotation.
if ( Mathf.Abs( CorrespondingGroundHit.sidewaysSlip ) > 2.0 ) {
if ( SlipPrefab ) {
Instantiate( SlipPrefab, CorrespondingGroundHit.point, Quaternion.identity );
}
}
}
I double checked the referencing of the wheel colliders and the car wheels and did the same with PlayerCar_Script.js and the Car references at my project in Unity, but I still got the error message. Hope you can help me with this. Thanks!
Oh, and I changed var GearRatio : float[]; to private var GearRatio : float[]; at the 8th line in the PlayerCar_Script.js, because I was getting “IndexOutOfRangeException: Array index is out of range.” error message.