Error when adding new task:

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

//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");
    //button.edit
    var editButton = document.createElement("button");
    //button.delete
    var deleteButton = document.createElement("button");

    //Each element needs modifing
    checkBox.type = "input";
    label.innerText = taskString;
    editButton.innerText = "Edit";
    editButton.className = "edit";
    deleteButton.innerText = "Delete";
    deleteButton.className = "delete";


    //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 new list item with the text from #new-task:
    var listItem = createNewTaskElement("Some new task");

    //Append listItem to incompleteTasksHolder
    incompleteTasksHolder.appendChild(listItem);
    bindTaskEvents(listItem, taskCompleted);

}


//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
}

//Delete an existing task
var deleteTask = function() {
    console.log("Delete task...");
    var listItem = this.parentNode;
    var ul = listItem.parentNode;

    //Remove the parent list item from the ul
    ul.removeChild(listItem);
}

//Mark a task as complete
var taskCompleted = function() {
    console.log("Task complete...");
    //Append the task list to the #completed-tasks
    var listItem = this.parentNode;
    completeTasksHolder.appendChild(listItem);
    bindTaskEvents(listItem, taskIncomplete);
}

//Mark a task as incomplete
var taskIncomplete = function() {
    console.log("Task incomplete...");
    //Append the task list to the #incomplete-tasks
    var listItem = this.parentNode;
    incompleteTasksHolder.appendChild(listItem);
    bindTaskEvents(listItem, taskCompleted);
}

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


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 the editTask to edit button
        editButton.onclick = editTask;

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

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


//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 < completeTasksHolder.children.length; i++) {
        //bind events to list item's children (taskIncomplete)
        bindTaskEvents(completeTasksHolder.children[i], taskIncomplete);
}