JS UI on click change text filed to active

Question

The JS snippet is from a Photoshop UI. Can someone tell me why the text field does not display active when the user ticks on the checkbox1? I have tried the onClick and onChange events without success. Thank you!

  dialog.checkbox1 = panel3.add("checkbox", undefined, undefined, {name: "checkbox1"}); 
  dialog.checkbox1.text = "Background Copy";
  dialog.checkbox1.value = checkbox1;
  checkbox1.onClick = function() {
    if (checkbox1 == true){
      dialog.edittext1.active = true;
    }
  }

  dialog.edittext1 = panel3.add('edittext {properties: {name: "edittext1"}}'); 
  dialog.edittext1.text = "Background copy"; 
  dialog.edittext1.alignment = ["fill","top"];
  dialog.edittext1.onChange = function() {
    if (dialog.edittext1.text == "") {
      alert("The text field can not be empty.");
    }
    else {
      win.hide();
    }
    dialog.edittext1.text = "Background copy"
  }