NullReferenceExeption help

Hi, everytime i place a ThirdPersonController on my person model i get a NullReferenceExeption message and i'm not sure why. This is the message: NullReferenceException

ThirdPersonController.UpdateSmoothedMovementDirection () (at Assets/Standard Assets/Character Controllers/Sources/Scripts/ThirdPersonController.js:105) ThirdPersonController.Update () (at Assets/Standard Assets/Character Controllers/Sources/Scripts/ThirdPersonController.js:320)

And this is the line of code that the error happens on:

var cameraTransform = Camera.main.transform;

Please help me

Camera.main is a property function which looks for the first active object with an enabled camera tagged as MainCamera. Make sure you have added the tag to a camera in your scene, and that it's enabled