Cannot set property 'onclick' of null

//Problem: User interactivity 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");
    //input (text)
    var editInput = document.createElement("input"); //checkbox
    //button.edit
    var editButton = document.createElement("button");
    //button.delete
    var deleteButton = document.createElement("button");

    //Each element needs modifying

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

    editButton.innerText = "Edit";
    editButton.className = "edit";
    deleteButton.innerText = "Delete";
    deleteButton.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 new list item with the text from #new-task:
    var listItem = createNewTaskElement(taskInput.value);

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

    taskInput.value = "";

}


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

var listItem = this.parentNode;

var editInput = listItem.querySelector("input[type=text");
var label = listItem.querySelector("label");

var containsClass = listItem.classList.contains("editMode");                                                                        
    //if the class of the parent is .editMode
    if(containsClass) {
    //Switch from .editMode
    //label text become the input's value
        label.innerText = editInput.value;
    } else {
    //switch to .editMode
    //input value becomes the label's text
    editInput.value = label.innerText;
}

//toggle .editMode on the list item
listItem.classList.toggle("editMode");
}

//Delete an existing task
var deleteTask = function() {
    console.log("Delete task...");
        //Remove the parent list item from the ul
    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 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 incompleted...");
        //Append the task list item to the #incompleted-tasks
    var listItem = this.parentNode;
    incompleteTasksholder.appendChild(listItem);
    bindTaskEvents(listItem, taskCompleted);
}

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 the editTask to edit button
    editButton.onclick = editTask;

        //bind the 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;

//var ajaxRequest = function() {
    //console.log("AJAX request");
//}
//addButton.onclick = ajaxRequest;

//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 completedTasksholder ul list items
                                     for(var i = 0; i < incompleteTasksholder.children.length; i++) {
        //bind events to list item's children (taskIncomplete)
        bindTaskEvents(incompleteTasksholder.children[i], taskIncomplete);
}