Unity Car Tutorial Error

Hello, I followed the Unity Car Tutorial and instead of the car I added my own model. It gave some errors when I ran it:

Error 1: NullReferenceException: Object reference not set to an instance of an object Car.CalculateState () (at Assets/Scripts/JavaScripts/Car.js:502) Car.FixedUpdate () (at Assets/Scripts/JavaScripts/Car.js:105)

Error 2: NullReferenceException: Object reference not set to an instance of an object Car.UpdateWheelGraphics (Vector3 relativeVelocity) (at Assets/Scripts/JavaScripts/Car.js:323) Car.Update () (at Assets/Scripts/JavaScripts/Car.js:95)

The actual car tutorial ran properly. I couldn't figure out the problem. Any solution would be greatly appreciated.