Can't find source of trigger Null Reference Exception

I have a box (non-trigger, kinematic rigidbody) that passes through different triggers (also set to kinematic rigidbody). The programming seems to function fine, but I keep getting a Null Reference Exception error, even though I've checked several times to make sure all Inspector-set variables have been dragged and dropped in place, and the right tags and scripts are assigned to the objects. Two of my three trigger types work fine ("slicks" and "clouds" are good, "gates" give the error), but they are set up the same way and compiling returns no errors.

The problem line is in the box script: if (triggerEnter.GetComponent().isActive == true)

Commenting out the line also makes the errors disappear.

Tests to check that something is returned on triggering come back correct, so I don't understand why I'm getting this error. I'm still new to C#. Does anyone know what the problem could be? Here are the scripts:

BOX SCRIPT

  using UnityEngine;
     using System.Collections;
     
     public class Box_Script : MonoBehaviour {
         
             
     public bool normalTimeSpeed = true; // has the time scale been changed?
     public float timeFast = 1.5f;        // controls time scale alterations
     public float timeSlow = 0.75f;
                   
     //========================================
         
         void Update () 
         {    
 
         
     // GATE TRIGGERS
         
         void OnTriggerExit(Collider triggerObject)
         {        
             if(triggerObject.tag == "Gate" && triggerObject.GetComponent<GatesGeneral>().isActive == true)
             {    
                 print (triggerObject.GetComponent<GatesGeneral>().isActive);//TEST. PRINTS "TRUE" LIKE IT SHOULD
                 
                 triggerObject.GetComponent<GatesGeneral>().isActive = false;        // prevent repeated triggering
             }
         }
         
     // TIME CONTROL TRIGGERS
         
         void OnTriggerEnter(Collider triggerEnter)
                     
         {        
             if (triggerEnter.GetComponent<Speeders>().isActive == true)    // PROBLEM LINE
             {    
                 
                 if(triggerEnter.tag == "Slick")                                        // Slicks speed up time
                 
                 {        
                     triggerEnter.GetComponent<Speeders>().isActive = false;            // prevent repeated triggering
                     
                     if (normalTimeSpeed == true)                                    // time is normal
                     {
                         Time.timeScale = timeFast;                                    // speed up time
                         normalTimeSpeed = false;
                     }
                                                                 
                     else                                                            // time was already slow                                                            
                     {
                         Time.timeScale = 1;                                            // return to normal
                         normalTimeSpeed = true;
                         
                     }
                 }
                 else if(triggerEnter.tag == "Cloud")                                // Clouds slow down time
                 {
                     
                     triggerEnter.GetComponent<Speeders>().isActive = false;            // prevent repeated triggering
                     
                     if (normalTimeSpeed == true)                                    
                     {
                         Time.timeScale = timeSlow;
                         normalTimeSpeed = false;
                     }
                 
                     else                                                                         
                     {
                         Time.timeScale = 1;
                         normalTimeSpeed = true;
                     }
                 }
             }
         }
     }

GATE SCRIPT

 using UnityEngine;
 using System.Collections;
 
 public class GatesGeneral : MonoBehaviour {
     
     
 public bool isActive = true;    // Item is active. Set by Box script to block repeated triggering (such as for speed modifiers)    
 public Material gateNormalRef;    // normal gate material (this variable for Lerp)
 public Material gateYesRef;        // material for a gate that has been successfully passed thru
 private float timer = 0;        // for Lerp
     
     void Update()
     {
         if (tag == "Gate" && isActive == false)    // Item tagged Gate has been collided with
             {    
                 renderer.material.Lerp (gateNormalRef, gateYesRef, timer);    // turn green
                 timer += (3.5f * Time.deltaTime);
             }    
     }
 }

CLOUD/SLICK SCRIPT

 using UnityEngine;
 using System.Collections;
 
 public class Speeders : MonoBehaviour {
 
     
 public bool isActive = true;    // Item is active. Set by Box script to block repeated triggering (such as for speed modifiers)
 public Material slickRef;
 public Material cloudRef;    
 public Material nullRef;
 private float timer = 0;        // for Lerp
     
     
     void Update () 
     {
         
     // MATERIAL LERPS
         
             if (tag == "Slick" && isActive == false)
         {    
             renderer.material.Lerp (slickRef, nullRef, timer);            // fade out
             timer += (0.25f * Time.deltaTime);
         }
         
             if (tag == "Cloud" && isActive == false)
         {    
             renderer.material.Lerp (cloudRef, nullRef, timer);            // fade out
             timer += (0.75f * Time.deltaTime);
         }
     }
 }