Mystery Uncaught TypeError: Cannot read property 'children' of null

    //set the click handler to the add task function
    //call the function when the user interacts with the button
    addButton.onlick = addTask;

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

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

    //Problem: User interaction doesn't provide desired results.
    //Solution: Add interactivity so the user can manage daily tasks.

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

    //add a new task
    var addTask = function(){
        console.log("Add Task...");
        //when a button is pressed create a new task
        //create a new list item with the text from #new-task:
            //create input with a checkbox
            //input (checkbox)
            //label
            //input (text)
            //button.edit
            //button.delete
            //Each element has to be modified and appended
    }

    //edit an existing task
    var editTask = function(){
        console.log("Edit Task...");
        //if the class of the parent is in .editMode
            //switch from .editMode
            //the label text becomes input's the value
        //else
            //switch to .editMode
            //the label value becomes label's the 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 (li) list item from the (ul)unordered list
    }

    //mark a task as complete
    var taskCompleted = function(){
        console.log("Task Complete ...");
        //when 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 checkbox is unchecked
            //append the task list item to the #incompleted-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 the delete button
            deleteButton.onclick = deleteTask;

            //bind checkboxEventHandler to checkbox
            checkBox.onchange = checkBoxEventHandler;
    }


    //set the click handler to the add task function
    //call the function when the user interacts with the button
    addButton.onlick = addTask;

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

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