NullReferenceException: Object reference not set to an instance of an object LastPlayerSighting.Awake () (at Assets/LastPlayerSighting.cs:23)

Here is my script I was working with the stealth project but it gives me that error when playing the music.

 using UnityEngine;
 using System.Collections;
 
 public class LastPlayerSighting : MonoBehaviour
 {
     public Vector3 position = new Vector3(1000f, 1000f, 1000f);         // The last global sighting of the player.
     public Vector3 resetPosition = new Vector3(1000f, 1000f, 1000f);    // The default position if the player is not in sight.
     public float lightHighIntensity = 0.25f;                            // The directional light's intensity when the alarms are off.
     public float lightLowIntensity = 0f;                                // The directional light's intensity when the alarms are on.
     public float fadeSpeed = 7f;                                        // How fast the light fades between low and high intensity.
     public float musicFadeSpeed = 1f;                                   // The speed at which the 
     
     
     private AlarmLight alarm;                                           // Reference to the AlarmLight script.
     private Light mainLight;                                            // Reference to the main light.
     private AudioSource panicAudio;                                     // Reference to the AudioSource of the panic msuic.
     private AudioSource[] sirens;                                       // Reference to the AudioSources of the megaphones.
     
     
     void Awake ()
     {
         // Setup the reference to the alarm light.
         alarm = GameObject.FindGameObjectWithTag(Tags.alarm).GetComponent<AlarmLight>();
         
         // Setup the reference to the main directional light in the scene.
         mainLight = GameObject.FindGameObjectWithTag(Tags.mainLight).light;
         
         // Setup the reference to the additonal audio source.
         panicAudio = transform.Find("secondaryMusic").audio;
         
         // Find an array of the siren gameobjects.
         GameObject[] sirenGameObjects = GameObject.FindGameObjectsWithTag(Tags.siren);
         
         // Set the sirens array to have the same number of elements as there are gameobjects.
         sirens = new AudioSource[sirenGameObjects.Length];
         
         // For all the sirens allocate the audio source of the gameobjects.
         for(int i = 0; i < sirens.Length; i++)
         {
             sirens[i] = sirenGameObjects[i].audio;
         }
     }
     
     
     void Update ()
     {
         // Switch the alarms and fade the music.
         SwitchAlarms();
         MusicFading();
     }
     
     
     void SwitchAlarms ()
     {
         // Set the alarm light to be on or off.
         alarm.alarmOn = position != resetPosition;
         
         // Create a new intensity.
         float newIntensity;
         
         // If the position is not the reset position...
         if(position != resetPosition)
             // ... then set the new intensity to low.
             newIntensity = lightLowIntensity;
         else
             // Otherwise set the new intensity to high.
             newIntensity = lightHighIntensity;
         
         // Fade the directional light's intensity in or out.
         mainLight.intensity = Mathf.Lerp(mainLight.intensity, newIntensity, fadeSpeed * Time.deltaTime);
         
         // For all of the sirens...
         for(int i = 0; i < sirens.Length; i++)
         {
             // ... if alarm is triggered and the audio isn't playing, then play the audio.
             if(position != resetPosition && !sirens[i].isPlaying)
                 sirens[i].Play();
             // Otherwise if the alarm isn't triggered, stop the audio.
             else if(position == resetPosition)
                 sirens[i].Stop();
         }
     }
     
     
     void MusicFading ()
     {
         // If the alarm is not being triggered...
         if(position != resetPosition)
         {
             // ... fade out the normal music...
             audio.volume = Mathf.Lerp(audio.volume, 0f, musicFadeSpeed * Time.deltaTime);
             
             // ... and fade in the panic music.
             panicAudio.volume = Mathf.Lerp(panicAudio.volume, 0.8f, musicFadeSpeed * Time.deltaTime);
         }
         else
         {
             // Otherwise fade in the normal music and fade out the panic music.
             audio.volume = Mathf.Lerp(audio.volume, 0.8f, musicFadeSpeed * Time.deltaTime);
             panicAudio.volume = Mathf.Lerp(panicAudio.volume, 0f, musicFadeSpeed * Time.deltaTime);
         }
     }
 }