Show object with tag after button press

Hi, I look in many posts here and try lot of options but nothing works. I have spaceship with script. It have child with tag "Plomien" (its Flame in Polish). I want to show that Flame when I go forward. Pls help. One by one i get error "NullReferenceExeption: Object Reference not set to an instance of an object".

 using UnityEngine;
 using System.Collections;
 
 public class Sterowanie : MonoBehaviour {
 
     public float predkosc;
     public float maksymalnaPredkosc = 10f;
     private float predkoscSkretu;
     private float predkoscZwrotu;
     private GameObject plomien;
 
     // Use this for initialization
     void Start () {
         predkosc = 0;
         predkoscSkretu = 0.2f;
         predkoscZwrotu = 0.2f;
         rigidbody.useGravity = false;
     //    GameObject.FindWithTag("Plomien").SetActive(true);
     //    plomien.activeSelf;
     }
 
     void FixedUpdate() 
     {
         //stala predkosc zalezna od zmiennej predkosc
         transform.Translate (0, 0, predkosc);
         //przyśpieszanie i maksymalna predkosc
         if (Input.GetKey(KeyCode.A) && predkosc < 10f) {
             Debug.Log("dzialaA");
             predkosc = predkosc+0.1f;

//HERE IS MY PROBLEM. PLS HELP!

             plomien = GameObject.FindGameObjectWithTag("Plomien");
             MeshRenderer p = plomien.GetComponent<MeshRenderer>();
             p.enabled = true;


         //    GameObject[] f1 = GameObject.FindGameObjectsWithTag("Plomien"); 
         //    foreach(GameObject f in f1)
         //    {
         //        f.GetComponent<MeshRenderer>().enabled = true;
         //    }
         }
         //zwalnianianie i predkosc minimalna
         if (Input.GetKey(KeyCode.Z) && predkosc > 0f) {
             Debug.Log("dzialaZ");
             predkosc = predkosc-0.1f;
         }
 
         //obrót w góre i dół zrobić dwoma punktami. Niech dziób unosi się w góre, a tył statku w dół
 
         if (Input.GetKey(KeyCode.UpArrow)) {
             transform.Rotate(Vector3.left * predkoscSkretu);
         }
         
         if (Input.GetKey(KeyCode.DownArrow)) {
             transform.Rotate(Vector3.right * predkoscSkretu);
         }
 
         if (Input.GetKey(KeyCode.LeftArrow)) {
             transform.Rotate(Vector3.forward * predkoscSkretu);
             transform.Rotate(Vector3.down * predkoscZwrotu);
         }
 
         if (Input.GetKey(KeyCode.RightArrow)) {
             transform.Rotate(Vector3.back * predkoscSkretu);
             transform.Rotate(Vector3.up * predkoscZwrotu);
         }
     }
     
     // Update is called once per frame
     void Update () {
 
     }
 }