help I know I'm over looking something

I'm new to Unity and have been working with the car tutorial. I have couple a months exp using Blender and just found Unity. It's great but I've run in the a problem with the java script. I imported in a car model I made in blender ( yea found out about axis the hard way ) set it up just as the tutorial instructed and I keep getting ((( NullReferenceException:Object reference not set to an instance of an object))) it refers to line 502 of the car script in the tutorial ####### if(w.collider.isGrounded)########## I've run out of ideas Any help would be great.