Uncaught TypeError: Cannot read property 'querySelector' of undefined + console shows add task when editing???

// Problem: user interaction doesnt provide desired results.
// Solution: Add interactivity so the user can manage daily tasks 

var taskInput = document.getElementById("new-task"); //new-task
var addButton = document.getElementsByTagName("button")[0]; //first-button
var incompleteTasksHolder = document.getElementById("incomplete-tasks");//incomplete-tasks
var completedTasksHolder = document.getElementById("completed-tasks"); // completed-tasks

//Add a new task 
var addTask = function () {
  console.log("Add task..");
  //When the button is pressed 
  //Creat a new list item with the text from #new-task:
    //input checkbox
    //label
    //input (text)
    //button.edit
    //button.delete
    //Each elements, needs to be modified and appended

}
//Edit an existing task
var editTask = function (){
  console.log("Edit task...");
  //When the Edit button is pressed
    //If the class of the parent is .editMode
      //Switch  from .editMode
      //label text become the input's value 
    //else
      //Switch to .editmode
      //input value becomes the label's text

      //Toggle .editMode on the parent

}

//Delete an existing task 
var  deleteTask = function (){
  console.log("delete task...");
  //When the delete button is pressed 
    //Remove the parent list item from the unordered list (ul)

}

//Mark a task as complete
var taskCompleted = function(){
   console.log("Task complete..");
  //When the checkbox is checked
    //Append the task list item to the #completed-tasks
}

//Mark a task as incomplete
var taskIncomplete = function() {
   console.log("Task incomplete..");
  //When the checkbox is unchecked
    //Append the task list item to the #complete-tasks
}

var bindTaskEvents = function(taskListItem, checkBoxEventHandler) {
  console.log("Bind list items");
  //select taskListItem's children
  var checkBox = taskListItem.querySelector("input[type=checkbox]");
  var editButton = taskListItem.querySelector("button.edit") ;
  var deleteButton = taskListItem.querySelector("button.delete") ;

    //bind editTask to edit button 
    editButton.onclick = editTask;

    //bind deleteTask to delete button 
  deleteButton.onclick = deleteTask;

    //bind checkBoxEventHandler to the checkbox
    checkBox.onchange = addTask;
}

//Set the click handler to the addTask function 
addButton.onclick = addTask;

//cycle over the incompleteTasksHolder ul list items
for(var i = 0; i < incompleteTasksHolder.children.length; i++){
    //bind events to list item's children (taskCompleted)
    bindTaskEvents(incompleteTasksHolder.children[i], taskCompleted);
}


for(var i = 0; i < incompleteTasksHolder.children.length; i++){
   //bind events to list item's children (taskIncompleted)
  bindTaskEvents(completedTasksHolder.children[i], taskIncomplete);
}