Perform: Traversing Elements with querySelector. Console "Uncaught TypeError: Cannot read property 'children" error

//problem: user interaction doesnt provide desired results.
//solution: add interaction with the user to manage daily task.
var taskInput = document.getElementById("new-task"); //new-task
var addButton = document.getElementsByTagName("button")[0]; //first button
var incompleteTasksHolder = document.getElementById("incomplete-task"); //incomplete-task
var completedTasksHolder  = document.getElementById("completed-task"); //completed-task

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


//edit an existing task
var editTask = function (){
    console.log("Edit Task .....");

    //when edit button is pressed
        //if the class of parent is editMode
            //switch from .editMode
            //label text become the input value
        //else
            //switch to .editMode
            //input value becomes the labels text
        //Toggle .editMode  
}

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

//mark a task as complete
var taskCompleted = function(){
    console.log("TaskCompleted .....");
    //when the checkbox is checked
        //append the task list item to the "completed task"
}

//mark a task as incomplete
var taskIncomplete = function (){
    console.log("Task Incomplete .....");
    //when the checkbox is unchecked
        //append the task list item to the "incompleted task"
    }

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


//set click handler to the addTask functions
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);
} 
    //for each list item


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