object reference is not set to an instance of an object

Unity Engine
2

You have Rigidbody rb = GetComponent(); in Start. This makes the variable a method variable and not a global one. Which means when you check for rb in FixedUpdate, there is no value assigned to that rb. My guess is you have a global variable as well, so in start remove the first Rigidbody

rb = GetComponent();

Also, please use code tags Using code tags properly