UnassignedReferenceException: The variable SkidTrailPrefab of WheelEffects has not been assigned.
You probably need to assign the SkidTrailPrefab variable of the WheelEffects script in the inspector.
UnityEngine.Object.Instantiate[Transform] (UnityEngine.Transform original) (at C:/buildslave/unity/build/Runtime/Export/UnityEngineObject.cs:201)
UnityStandardAssets.Vehicles.Car.WheelEffects+<StartSkidTrail>c__Iterator0.MoveNext () (at Assets/Standard Assets/Vehicles/Car/Scripts/WheelEffects.cs:73)
UnityEngine.SetupCoroutine.InvokeMoveNext (IEnumerator enumerator, IntPtr returnValueAddress) (at C:/buildslave/unity/build/Runtime/Export/Coroutines.cs:17)
UnityEngine.MonoBehaviour:StartCoroutine(IEnumerator)
UnityStandardAssets.Vehicles.Car.WheelEffects:EmitTyreSmoke() (at Assets/Standard Assets/Vehicles/Car/Scripts/WheelEffects.cs:51)
UnityStandardAssets.Vehicles.Car.CarController:CheckForWheelSpin() (at Assets/Standard Assets/Vehicles/Car/Scripts/CarController.cs:286)
UnityStandardAssets.Vehicles.Car.CarController:Move(Single, Single, Single, Single) (at Assets/Standard Assets/Vehicles/Car/Scripts/CarController.cs:174)
UnityStandardAssets.Vehicles.Car.CarUserControl:FixedUpdate() (at Assets/Standard Assets/Vehicles/Car/Scripts/CarUserControl.cs:27)