NullReferenceException: Object reference not set to an instance of an object iShake.Update () (at Assets/scripts/iShake.js:32)

I am new to javascript. I am executing the following unity script. I am getting the NullReferenceException on line 32. I will greatly helpful if anyone could identify the problem.

 #pragma strict
 
 import ThinksquirrelSoftware.Fluvio.Emitters;
 
 var accelerometerUpdateInterval : float = 1.0 / 60.0;
 var lowPassKernelWidthInSeconds : float = 1.0;
 var shakeDetectionThreshold : float = 2.0;
 
 internal var emitter : FluidEmitter;
 
 private var lowPassFilterFactor : float = accelerometerUpdateInterval / lowPassKernelWidthInSeconds; 
 private var lowPassValue : Vector3 = Vector3.zero;
 private var acceleration : Vector3;
 private var deltaAcceleration : Vector3;
 
 function Start()
 {
     shakeDetectionThreshold *= shakeDetectionThreshold;
     lowPassValue = Input.acceleration;
 }
 
 function Update()
 {
     acceleration = Input.acceleration;
         lowPassValue = Vector3.Lerp(lowPassValue, acceleration, lowPassFilterFactor);
         deltaAcceleration = acceleration - lowPassValue;
     if (deltaAcceleration.sqrMagnitude >= shakeDetectionThreshold)
     {
         Debug.Log("Shake event detected at time "+Time.time);
     }  else {
         Debug.Log("Shake event NOT DETECTED ");
         **emitter.Emit();**
     }
 }