How to assign property value if not null, to object instance in TypeScript, Angular

I looping through array of object and creating instance of specific class, followed by assigning values accordingly. Everything working fine, expect I need to check if value does exist then only assign.

In my scenario, I have list of questions which may or may not have answer so to check and assign value if not null, I doing following code but throwing error.

value: questionsList[key].answer.length>0? null : questionsList[key].answer[0].answerValue

complete code

if(questionElementType=="textbox")
 {       

  var isAnswerExist = questionsList[key].answer.length;
  if(isAnswerExist>0) // this one does work
  {
    var ans = questionsList[key].answer[0].answerValue;
    console.log("answer  ", ans);
  }


   let _textBox = new TextboxQuestion({
     questionId: questionsList[key].questionId,
     questionElementType: questionsList[key].questionElementType[0].title,        
     questionType: questionsList[key].questionType,
     title:questionsList[key].title,
     displayId: questionsList[key].displayId,
     key: questionsList[key].questionId, 
     label: questionsList[key].title,
     value: questionsList[key].answer.length>0? null : questionsList[key].answer[0].answerValue,  // need help here
     required: true,
     order: 5
    }); 

    this.mappedQuestions.push(_textBox);

 }

Error 1

formGroup expects a FormGroup instance. Please pass one in.

   Example:


<div [formGroup]="myGroup">
  <input formControlName="firstName">
</div>

In your class:

this.myGroup = new FormGroup({
   firstName: new FormControl()
});

Error 2

TypeError: Cannot read property 'answerValue' of undefined