How to check if a method is not null?

Since I am relatively new to programming I don't now how to check if a method is not null. In the following code I need to check if the answers.get method is not null, how can I achieve this?

public class SharedViewModel extends ViewModel {


private HashMap<Integer, MutableLiveData<String>> answers = new HashMap<>();

public MutableLiveData<String> getAnswer(int questionId) {
    return answers.get(questionId);
}

public void setAnswer(int questionId, String answer) {
    // TODO make sure that answers.get is not null

    answers.get(questionId).setValue(answer);
 }
}