Null reference exception on Screenpointtoray (Multiplayer)

Hey all. In my multiplayer game of Hunter vs Hunted the hunted team have to complete objectives. I set up the following script for this...

 void Update () {
 
         if (Input.GetButtonDown("Fire2")){
  
             NetworkViewID viewID = Network.AllocateViewID();

             networkView.RPC("RaycastObjective",RPCMode.AllBuffered, viewID);
   
         }

     }
 
     [RPC]
 
     void RaycastObjective(NetworkViewID viewID){

         RaycastHit ObjectiveHit = new RaycastHit();
 
         Ray objRay = Camera.main.ScreenPointToRay(Input.mousePosition);
 
         if (Physics.Raycast(objRay, out ObjectiveHit, 400))
 
         {

           if (ObjectiveHit.transform.name == "IDCard")
 
             {
   
             currentobjective = 1;
 
             Debug.Log("Hit Keycard!");  
 
             Destroy(IDCard);
   
             }
   
                 if (ObjectiveHit.transform.name == "Generator")
 
             {
 
                 if (currentobjective == 1)
 
                 {
 
                     
 
                 Debug.Log("Generator Activated!");
 
                 currentobjective = 2;
 

                 }
 

             }
   
         }

The problem is that when the Hunter team spawns they get this error...

NullReferenceException UnityEngine.Camera.ScreenPointToRay (Vector3 position) (at C:/BuildAgent/work/ea95e74f6e5f192d/Runtime/ExportGenerated/Editor/UnityEngineCamera.cs:379) Obj_Sketchup.RaycastObjective (NetworkViewID viewID) (at Assets/Scripts/MapScripts/Obj_Sketchup.cs:50) UnityEngine.NetworkView:RPC(String, RPCMode, Object[]) Obj_Sketchup:Update() (at Assets/Scripts/MapScripts/Obj_Sketchup.cs:32)

From what this error says it means that the lines responsible for the error are...

 networkView.RPC("RaycastObjective",RPCMode.AllBuffered, viewID);

and...

 Ray objRay = Camera.main.ScreenPointToRay(Input.mousePosition);

I can't for the life of me work out why I would be getting a null reference exception error? I have all cameras tagged properly and it seems odd that only one team are getting the error. What would you suggest for a work around?