Can't Call Function From Other Script

I did try this, but i get a error. NullReferenceException: Object reference not set to an instance of an object TextControl.Update () (at Assets/Scripts/QuestionsScripts/TextControl.cs:43)

here is apart of my code

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class TextControl : MonoBehaviour
 {
     List<string> questions = new List<string>() {"1","2","3","4","5","6","7","8","9","10","11"}; //Questions
     List<string> correctAnswer = new List<string>() {"4", "1", "3", "4", "1", "2", "3", "4", "1", "2", "3" }; //Correct Awnser 
     public Transform resultObj;
     public static string selectedAnswer;
     public static string choiceSelected = "n";
     public static int randQuestion=-1;
     public Transform correctAnswerObj;
     public PlayerStone wrongAnswer;
     // Start is called before the first frame update
     void Start()
     {
        // GetComponent<TMPro.TextMeshProUGUI>().text = questions[0];
     }
 
     // Update is called once per frame
     void Update()
     {
         if (randQuestion == -1)
         {
             randQuestion = Random.Range(0, 10);
         }
         if (randQuestion > -1)
         {
             GetComponent<TMPro.TextMeshProUGUI>().text = questions[randQuestion];
         }
         //Debug.Log(questions[randQuestion]);
         if (choiceSelected == "y")
         {
             choiceSelected = "n";
             if (correctAnswer[randQuestion]==selectedAnswer)//checks slected answer is correct
             {
                 resultObj.GetComponent<TMPro.TextMeshProUGUI>().text = "Correct Answer!!" ;//correct answer
             }
             else
             {
                 BackToMenu.resetCorrectAnswer = "n";
                 wrongAnswer.ReturnToStorage();
                 resultObj.GetComponent<TMPro.TextMeshProUGUI>().text = "Incorrect Answer!!";//wrong answer
 
                 if (correctAnswer[randQuestion] == "1")
                 {
                     correctAnswerObj.GetComponent<Transform>().position = new Vector3(140, 290, 0);
                 }
                 if (correctAnswer[randQuestion] == "2")
                 {
                     correctAnswerObj.GetComponent<Transform>().position = new Vector3(630, 290, 0);
                 }
                 if (correctAnswer[randQuestion] == "3")
                 {
                     correctAnswerObj.GetComponent<Transform>().position = new Vector3(630, 170, 0);
                 }
                 if (correctAnswer[randQuestion] == "4")
                 {
                     correctAnswerObj.GetComponent<Transform>().position = new Vector3(140, 170, 0);
                 }
             }
         }
     }
 }