Uncaught error. Cannot read property of children

//problem
//user interaction doesn't provide results

//solution
//add interactivity

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

//add a new task
var addTask = function(){
    console.log("add Task...");
    //when the button is pressed
    //create a new list item with the text from #new-task
        //create an input check box
        //create label
        //create input(text)
        //create button (.edit)
        //create button (.delete)
        //each element, need to be modified and appended

}


//edit an existing task (toggle .editMode)
var editTask = function(){
    console.log("edit Task...");
    //when edit 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 = label text
}




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


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



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

}

var bindTaskEvents = function(taskListItem, checkBoxEventHandler){
    console.log('bind list events');
    //select taskListItems 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 addTask function
addButton.onclick = addTask;


//cycle over incompleteTasksHolder 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 completedTasksHolder 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);
}