I am getting a Uncaught TypeError for: var checkBox = taskListItem.querySelector("input[type=checkbox]");

//Problem: User interaction does not provide the desired results. 
//Solution: Add interactivity so the user can manage daily tasks. 

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

//Add a new task.
var addTask = function() {
  console.log("Add tasks...");
  //When the button is pressed:
  //Create a new list item with the text from #new-task
    //Input (checkbox)
    //Label
    //Input (text)
    //Button.edit. 
    //Button.delete.
    //Each elemts will need to be modified and appended. 
}

//Edit an existing task. 
var editTask = function() {
  console.log("Edit tasks...");
  //When the edit button is pressed:
    //If the class of the parentis .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 tasks...");
  //When the delete button is pressed:
    //Remove the parent list item from the ul. 
}

//Mark a task as complete. 
var taskCompleted = function() {
  console.log("Tasks 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 #incomplete-tasks.
}

var bindTaskEvents = function (taskListItem, checkBoxEventHandler) {
  console.log("Bind list item events");
  //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 check box.
    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++) {
    //Bind events to list item's children (taskCompleted).
    bindTaskEvents(incompleteTasksHolder.children[i], taskCompleted);
}

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