Uncaught TypeError: Failed to execute 'appendChild' on 'Node': parameter 1 is not of type 'Node'.app.js:51 addTask

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

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

//New Task List Item
var createNewTaskElement = function(taskstring){
    //Create List Item
    var listItem = document.createElement("li");
    //input checkbox
    var checkBox = document.createElement("input"); //checkbox
    //label
    var label = document.createElement("label"); //label
    //input text
    var editInput = document.createElement("input"); //text
    //button.edit
    var editButton = document.createElement("button"); //edit
    //buton.delete
    var deleteButton = document.createElement("button"); //delete
    //Each element needs to be modified

  checkBox.type = "checkbox";
  editInput.type = "text";

  editButton.innerText = "Edit";
  editButton.className = "edit";
  deleteButton.innerText = "Delete";
  deleteButton.className = "delete";



    //Each element needs to be appended
  listItem.appendChild(checkBox);
  listItem.appendChild(label);
  listItem.appendChild(editInput);
  listItem.appendChild(editButton);
  listItem.appendChild(deleteButton);
}

//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 the #new-task:
  var listItem = createNewTaskElement(taskInput.value);
  //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 to label's text
    //Toggle .editMode
}
//Delete an existing task
var deleteTask = function(){
    console.log("Delete task...");
  //When the delete button is pressed
  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("Complete task...");
    //Append the task list item to the #completed-tasks
  var listItem = this.parentNode;
  completedTasksHolder.appendChild(listItem);
  bindTaskEvents(listItem, taskinComplete);
}

//Mark a task as incomplete
var taskinComplete = function(){
  console.log("Task incomplete...");
  //Append the task list item 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 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 delete button
  deleteButton.onclick = deleteTask;
    //bind taskCompleted to checkbox
  checkbox.onchange = checkboxEventHandler;
}

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

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