(Javascript) Object reference not set to an instance of an object

I am making a simple car game. I have trouble making the AI though. Here is my script:

 var FrontLeftWheel : WheelCollider;
 var FrontRightWheel : WheelCollider;
 var BackLeftWheel : WheelCollider;
 var BackRightWheel : 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 DifferentialRatio : float = 3.1;
 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;
 
 // Here's all the variables for the AI, the waypoints are determined in the "GetWaypoints" function.
 // the waypoint container is used to search for all the waypoints in the scene, and the current
 // waypoint is used to determine which waypoint in the array the car is aiming for.
 var waypointContainer : GameObject;
 private var waypoints : Array;
 private var currentWaypoint : int = 0;
 
 // input steer and input torque are the values substituted out for the player input. The 
 // "NavigateTowardsWaypoint" function determines values to use for these variables to move the car
 // in the desired direction.
 private var inputSteer : float = 0.0;
 private var inputTorque : 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 = -.45;
     rigidbody.centerOfMass.z = -0.15;
     
     // Call the function to determine the array of waypoints. This sets up the array of points by finding
     // transform components inside of a source container.
     GetWaypoints();
 }
 
 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;
     
     // Call the funtion to determine the desired input values for the car. This essentially steers and
     // applies gas to the engine.
     NavigateTowardsWaypoint();
     
     // Compute the engine RPM based on the average RPM of the two wheels, then call the shift gear function
     EngineRPM = Mathf.Abs(BackLeftWheel.rpm + BackRightWheel.rpm)/2 * GearRatio[CurrentGear] * DifferentialRatio;
     if ( EngineRPM>10000) {EngineRPM =10000;}
     if ( EngineRPM<0) {EngineRPM =0;}
     ShiftGears();
 
     // set the audio pitch to the percentage of RPM to the maximum RPM plus one, this makes the sound play
     // up to twice it's pitch, where it will suddenly drop when it switches gears.
     audio.pitch = Mathf.Abs(EngineRPM / MaxEngineRPM) + 0.5 ;
     // this line is just to ensure that the pitch does not reach a value higher than is desired.
     if ( audio.pitch > 1.5 ) {
         audio.pitch = 1.5;
     }
     
     // finally, apply the values to the wheels.    The torque applied is divided by the current gear, and
     // multiplied by the calculated AI input variable.
     //FrontLeftWheel.motorTorque = EngineTorque / GearRatio[CurrentGear]*DifferentialRatio * inputTorque;
     //FrontRightWheel.motorTorque = EngineTorque / GearRatio[CurrentGear]*DifferentialRatio * inputTorque;
     BackLeftWheel.motorTorque = -EngineTorque / GearRatio[CurrentGear]*DifferentialRatio * inputTorque;
     BackRightWheel.motorTorque = -EngineTorque / GearRatio[CurrentGear]*DifferentialRatio * inputTorque;
     
     // the steer angle is an arbitrary value multiplied by the calculated AI input.
     FrontLeftWheel.steerAngle = 35 * inputSteer;
     FrontRightWheel.steerAngle = 35 * inputSteer;
 }
 
 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 ( BackLeftWheel.rpm * GearRatio[i]*DifferentialRatio < MaxEngineRPM ) {
                 AppropriateGear = i;
                 break;
             }
         }
         
         CurrentGear = AppropriateGear;
     }
     
     if ( EngineRPM <= MinEngineRPM ) {
         AppropriateGear = CurrentGear;
         
         for ( var j = GearRatio.length-1; j >= 0; j -- ) {
             if ( BackLeftWheel.rpm * GearRatio[j]*DifferentialRatio > MinEngineRPM ) {
                 AppropriateGear = j;
                 break;
             }
         }
         
         CurrentGear = AppropriateGear;
     }
 }
 
 function GetWaypoints () {
     // Now, this function basically takes the container object for the waypoints, then finds all of the transforms in it,
     // once it has the transforms, it checks to make sure it's not the container, and adds them to the array of waypoints.
     var potentialWaypoints : Array = waypointContainer.GetComponentsInChildren( Transform );
     waypoints = new Array();
     
     for ( var potentialWaypoint : Transform in potentialWaypoints ) {
         if ( potentialWaypoint != waypointContainer.transform ) {
             waypoints[ waypoints.length ] = potentialWaypoint;
         }
     }
 }
 
 function NavigateTowardsWaypoint () {
     // now we just find the relative position of the waypoint from the car transform,
     // that way we can determine how far to the left and right the waypoint is.
     var RelativeWaypointPosition : Vector3 = transform.InverseTransformPoint( Vector3( 
                                                 waypoints[currentWaypoint].position.x, 
                                                 transform.position.y, 
                                                 waypoints[currentWaypoint].position.z ) );
                                                                                 
                                                                                 
     // by dividing the horizontal position by the magnitude, we get a decimal percentage of the turn angle that we can use to drive the wheels
     inputSteer = RelativeWaypointPosition.x / RelativeWaypointPosition.magnitude;
     
     // now we do the same for torque, but make sure that it doesn't apply any engine torque when going around a sharp turn...
     if ( Mathf.Abs( inputSteer ) < 1.0 ) {
         inputTorque = RelativeWaypointPosition.z / RelativeWaypointPosition.magnitude - Mathf.Abs( inputSteer );
     }else{
         inputTorque = 0.0;
     }
     
     // this just checks if the car's position is near enough to a waypoint to count as passing it, if it is, then change the target waypoint to the
     // next in the list.
     if ( RelativeWaypointPosition.magnitude < 30 ) {
         currentWaypoint ++;
         
         if ( currentWaypoint >= waypoints.length ) {
             currentWaypoint = 0;
         }
     }
     
 }

On line 55 it gives me the error: Object reference not set to an instance of an object. How to I fix this?