Unassigned Reference Exception?

I've been trying to code the player ship for a Lunar Lander style game, however, the script is giving me three errors: The variable rightThruster is unassigned, the variable leftThruster is unassigned, and the variable rightThruster is unassigned (Again).

As far as I can tell, all of the variables have been properly assigned. Could anyone tell me what I'm missing, and explain it a bit so it doesn't happen again?

 #pragma strict
 
 var downThruster: ParticleEmitter;
 var topThruster: ParticleEmitter;
 var leftThruster: ParticleEmitter;
 var rightThruster: ParticleEmitter;
 
 function Start () 
 {
     
 }
 
 function Update ()
 {
     //Moving right
     if(Input.GetAxis("Horizontal") > 0)
     {
         leftThruster.emit = true;
         rightThruster.emit = false;
         rigidbody.AddForce(10,0,0);
     }
     //Moving left
     if(Input.GetAxis("Horizontal") < 0)
     {
         rightThruster.emit = true;
         leftThruster.emit = false;
         rigidbody.AddForce(-10,0,0);
     }
     //Horizontal still
     if(Input.GetAxis("Horizontal") == 0)
     {
         rightThruster.emit = false;
         leftThruster.emit = false;
         rigidbody.AddForce(0,0,0);
     }
     //Moving up
     if(Input.GetAxis("Vertical") > 0)
     {
         downThruster.emit = true;
         topThruster.emit = false;
         rigidbody.AddForce(0,10,0);
     }
     //Moving down
     if(Input.GetAxis("Vertical") < 0)
     {
         topThruster.emit = true;
         downThruster.emit = false;
         rigidbody.AddForce(0,-10,0);
     }
     //Vertical still
     if(Input.GetAxis("Vertical") == 0)
     {
         downThruster.emit = false;
         topThruster.emit = false;
         rigidbody.AddForce(0,0,0);
     }
 }