NullReferenceException randomly occurs

I imagine it's in here:

 landedPlatformNumber = collision.gameObject.GetComponent<PlatformController> ().platformNumber;
          GameObject[] platforms = GameObject.FindGameObjectsWithTag ("Platform");
              for (var i = 0; i < platforms.Length; i++) {
                  if (platforms [i].GetComponent<PlatformController> ().platformNumber < landedPlatformNumber) {
                      platforms [i].GetComponent<PlatformController> ().DropPlatform ();
                  }
              }

There's a lot of improvement and null checking that should be done like so:

 PlatforController pc = collision.gameObject.GetComponent<PlatformController> ();
 if (pc != null)
 {
     landedPlatformNumber = pc.platformNumber;
     GameObject[] platforms = GameObject.FindGameObjectsWithTag ("Platform");

     for (var i = 0; i < platforms.Length; i++) 
     {
         PlaformController pc2 = platforms [i].GetComponent<PlatformController> ();
         if (pc2 == null)
             continue;

         if (pc2.platformNumber < landedPlatformNumber) 
         {
             pc2.DropPlatform ();
         }
     }
 }

A few notes:

• When using GetComponent always check that the component isn't null before accessing its members.

• Do everything you can to reduce calls to GetComponent because it is slow.

• Similarly reduce calls to Find methods because they are slow too.

• It would be better that this script is able to reference a List of PlatformControllers that have been created. That way you can check if collision.gameObject = platformControllerList[i].gameObject and also use that List to iterate through without the need of all these Find and GetComponent calls. Still check for nulls though before accessing members!