Tagging a prefab breaks my code?

This is a class I am using. SendActivity() is called when the player performs an action. It finds any objects in the scenes tagged as "Receiver", stores that as a temporary array and then sets a bool to true in each of those objects.

This was working exactly as intended for the longest time, until I created a new prefab. I changed nothing in this code or any related code, all I did was add a prefab and tagged it as "Receiver". Previously, I had two Prefabs tagged as such. This new prefab and the existing ones have the ActivityReceiver script attached. Now, whenever SendActivity() is called at all, it throws a Null Reference Exception. The prefab that was added isn't even instantiated in the scene. If I delete the prefab or change its tag back to Untagged, everything works again.

I am dumbfounded here, help please!?

 GameObject[] receivers;
 
     void FindReceivers(){
 
         receivers = GameObject.FindGameObjectsWithTag("Receiver");
     }
 
     public void SendActivity(){
 
         FindReceivers();
 
         for(int x=0; x<receivers.Length; x++){
 
             receivers[x].GetComponent<ActivityReceiver>().activityReceived = true;
 
         }
 
     }