Reference variable simultaneously null and full

I wrote a script that checks if the player is Moving and if the 'footstep' sound isn't playing. If this isn't true, it plays the footstep noise.

And it works. I run the scene, I move the character, footstep noises play, one after the other, until I stop.

Except, Unity has decided it doesn't work, and is spitting a null reference variable at me every single frame, referencing a variable that is grabbed from another script using a serialized field.

"NullReferenceException: Object reference not set to an instance of an object FootstepController.Update () (at Assets/Scripts/FootstepController.cs:17)"

It gets wierder. I re-wrote the script to remove the reference variable to the other script...and it started spitting this error at me:

"UnassignedReferenceException: The variable step of FootstepController has not been assigned. You probably need to assign the step variable of the FootstepController script in the inspector. FootstepController.Update () (at Assets/Scripts/FootstepController.cs:17)"

Except it was assigned in the inspector, and it was working! My character was making footstep noises as he walked around the test area.

What is happening?

The relevant code, omitting some variables off the top, by the way:

     public AudioSource step;
 
     public PlayerController playerController;
 
 
     void Update (){
             if (playerController.currentspeed > 0f && !(step.isPlaying)) {
             stepSelector();
         }    
     }
 
     void stepSelector(){
         d10 = Random.Range (0,9);
         step.clip = dirtSteps [d10];
         step.Play ();
     }
 }