title: how to set value in formArray
body: <p>I have got a formgroup named input form, having an formarray of topics. I want to change the value of an inputfield using a dynamically created filtered list. </p>
  
  <p>the filter function works fine: </p>
  
  <pre><code>filtertopic(idx: number) {
    const test = this.topics.value[idx].topic;
    if (test !== "") {
      this.onderdeel = "topics";
      this.queryselector = idx;
      this.filteredList = this.Topics.filter(function(el) {
        return el.toLowerCase().indexOf(test.toLowerCase()) &gt;-1;
      }.bind(this));
       } else { this.filteredList = [];
      }
      }
  </code></pre>
  
  <p>but the function handleBlur to change the value in the input field does not work</p>
  
  <pre><code>handleBlur() {
      console.log(this.selectedIdx);
      if (this.selectedIdx &gt; -1) {
        if (this.onderdeel =="topics") {
          this.topics.value[this.queryselector].topic.setValue(this.filteredList[this.selectedIdx]);
        } else {
          this.query = this.filteredList[this.selectedIdx];
        }
      }
      this.filteredList = [];
      this.selectedIdx = -1;
  }
  </code></pre>
  
  <p>I think it has to do with the </p>
  
  <blockquote>
    <p>this.topics.value[this.queryselector].topic.setValue(this.filteredList[this.selectedIdx]);</p>
  </blockquote>
  
  <p>to set the formcontrol value. Does anybody know the solution?</p>