Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' with topicName.setText(getTopicName); - nullpointerexception

Question

it shows an error on this line but i dont know what to do please explain and help
showing me a nullpoiterexception i dont know how to resolve it im new to this type of null poiterexception
topicName.setText(getTopicName);
MAIN ACTIVITY CODE:
public class MainActivity extends AppCompatActivity {
// total quiz time in minutes
private int totalTimeInMins = 1;
// Timer class object for countdown timer
private Timer quizTimer;
private int seconds = 0; // current countdown seconds
// questions array list
private List<QuestionsList> questionsLists;
// Current questions index position from questionsLists ArrayList.
private int currentQuestionPosition = 0;
// Options
private AppCompatButton option1, option2, option3, option4;
// next button
private AppCompatButton nextBtn;
// Total questions and main question TextView
private TextView question;
private TextView questions;
// selectedOption's Value. if user not selected any option yet then it is empty by default
private String selectedOptionByUser = "";
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
// initialize widgets from activity_main.xml file
final ImageView backBtn = findViewById(R.id.backBtn);
final TextView topicName = findViewById(R.id.topicName);
final TextView timer = findViewById(R.id.timer);
question = findViewById(R.id.question);
questions = findViewById(R.id.questions);
option1 = findViewById(R.id.option1);
option2 = findViewById(R.id.option2);
option3 = findViewById(R.id.option3);
option4 = findViewById(R.id.option4);
nextBtn = findViewById(R.id.nextButton);
// get Topic Name and User Name from StartActivity via Intent
final String getTopicName = getIntent().getStringExtra("selectedTopic");
// set Topic Name to TextView
topicName.setText(getTopicName);
// start quiz countdown timer
startTimer(timer);
// get questions from QuestionsBank class according to selectedTopicName and assign to questionsLists ArrayList
questionsLists = QuestionsBank.getQuestions(getTopicName);
it says theres something wrong with line '67' but idk what it is...
this one : topicName.setText(getTopicName);
error messgae says this:
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference
at com.learnoset.offlinequizapp.MainActivity.onCreate(MainActivity.java:67)