Uncaught TypeError: Cannot set property 'onclick' of null - bindTaskEvents @ app.js:117, and addTask @ app.js:61

/***************************
Variables
***************************/

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



/***************************
Functions
***************************/

//New Task List Item
var createNewTaskElement = function(taskString){
  //createlist item
  var listItem = document.createElement("li"); 
  //input (checkbox)
  var checkBox = document.createElement("input"); // checkbox
  //label
  var label = document.createElement("label");
  //input (text)
  var editInput = document.createElement("input"); // text
  //button.edit
  var editButton = document.createElement("button"); // edit
  //button.delete
  var deleteButton = document.createElement("button"); // delete

  //Each element need modifying
  checkBox.type = "checkbox";
  editInput.type = "text";
  editButton.innerText = "Edit";
  editButton.className = "edit";
  editButton.innerText = "Delete";
  editButton.className = "delete";
  label.innerText = taskString

  //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 list item with the text from #new-task
  var listItem = createNewTaskElement(taskInput.value);

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

//Edit an existing task
var editTask = function() {  //this is an eventHandler
  console.log("Edit task...");
  //When the edit button is pressed
    //Switch from .editMode
    //label text becomes the input's value
  //else
    //Switch to .editMode
    //input value becomes the label's text

  //toggle .editMode on the parent
}

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

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

var taskCompleted = function() { //this is an eventHandler
  console.log("Task complete...");
  //Append the text list item to the #completed-tasks
  var listItem = this.parentNode;
  completedTasksHolder.appendChild(listItem);
  bindTaskEvents(listItem, taskIncomplete);
}

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



/***************************
Bind Events
***************************/

var bindTaskEvents= function(taskListItem, checkBoxEventHandler) { /*this process is repeated to cycle over incompleteTaskHolder and completedTaskHolder so we created a function (DRY) */
  console.log("Bind list item"); 

  //select taskListItem 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 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 childeren's (taskCompleted) checkbox
  bindTaskEvents(incompleteTasksHolder.children[i], taskCompleted);
}
//cycle over completeTasksHolder ul list items
for(var i = 0; i < completedTasksHolder.children.length; i++) {
  //bind events to list item's childeren's (taskInompleted) checkbox
  bindTaskEvents(completedTasksHolder.children[i], taskIncomplete);
}
  //bind events to list item's childeren's (taskIncomplete) checkbox