Getting error: Uncaught TypeError: Cannot read property 'querySelector' of undefined

//Problem: user interaction doesn't 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

//New task list item
var createNewTaskElement = function(taskString) {

  //create list item
  var listItem = document.createElement("li");

  //input (checkbox)
  var checkBox = document.createElement("input");
    //label
  var label = document.createElement("label");
    //input (text)
  var editInput = document.createElement("input");
    //create button.edit
  var editButton = document.createElement("button");
    //creat button.delete
  var deleteButton = document.createElement("button");

  //each element needs modifying

  //each element needs appending
  listItem.appendChild(checkBox);
  listItem.appendChild(label);
  listItem.appendChild(editInput);
  listItem.appendChild(editButton);
  listItem.appendChild(deleteButton);

  return listItem;
}

//add a new task
var addTask = function() {
  console.log("Add task");
  //create a tnew list item with text from #new-task
  var listItem = createNewTaskElement("some new task");

  //append to incompleteTaskHolder
  incompleteTasksHolder.appendChild(listItem);
}

//edit existing task
var editTask = function() {
  console.log("Edit task");
  //when edit button is pressed
    //if the class of the parent is .editMode
      //switch from editMode
      //label text become the inputs value
    //else
      //switch to editMode
      //input value becomes labels text

    //toggle .editMode
}

//delete an existing task
var deleteTask = function() {
  console.log("delete task");
  //when delete button is pressed 
    //remove parent list item from the ul
}

//mark a task as complete
var taskCompleted = function() {
  console.log("task complete");
  //append the task list item to the #completed-tasks
  var listItem = this.parentNode;
  completedTasksHolder.appendChild(listItem);
}

//mark a task as incomplete
var taskIncomplete = function() {
  console.log("task incomplete");
  //append to #incomplete-tasks
  var listItem = this.parentNode;
  incompleteTasksHolder.appendChild(listItem);
}

var bindTaskEvents = function(taskListItem, checkBoxEventHandler) {
  console.log("Bind list item events..");
    //select list item'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 checkbox
  checkBox.onchange = checkBoxEventHandler;
}

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

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

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