Select correct answer in a quiz game

Currently working on a project that requires making a quiz game in unity with data coming from a firebase web api

I was able to retreive data an put them in text fields and buttons in the scene but i couldn't figure out a way to make the logic of the game, how to identify the correct answer and add score.

This is my answer code

         public void Answer()
 {
     if (isCorrect)
     {
         GetComponent<Image>().color = Color.green;
         Debug.Log("Correct Answer");
         theQuiz.correct();
     }
     else
     {
         GetComponent<Image>().color = Color.red;
         Debug.Log("Wrong Answer");
         theQuiz.wrong();
     }
 }

And this is my correct answer verification code

         var quest = JsonConvert.DeserializeObject<QuestionResponse>(response.downloadHandler.text);
     if (quest.documents.Count >= 0)
     {
         currentQuestion = Random.Range(0, quest.documents.Count);
         QuestionTxt.text = quest.documents[currentQuestion].fields.q.stringValue;
         Ch1Txt.text = quest.documents[currentQuestion].fields.ch1.stringValue;
         Ch2Txt.text = quest.documents[currentQuestion].fields.ch2.stringValue;
         Ch3Txt.text = quest.documents[currentQuestion].fields.ch3.stringValue;
         Ch4Txt.text = quest.documents[currentQuestion].fields.ch4.stringValue;

          Text c1 = Ch1Txt.GetComponent<Text>();

         totalQuestions = quest.documents.Count;

         c1.GetComponent<AnswerScriptApi>().isCorrect = true;

When i execute this a NullReferenceException: Object reference not set to an instance of an object appears