Cannot set property 'onchange' of null

//Problem User interaction doent give desired results
//Solution: Add interactivity


var taskInput = document.getElementById("new-task");// New Task
var addButton= document.getElementsByTagName("button")[0];//First BUtton
var incompleteTaskHolder = document.getElementById("incomplete-tasks");//İncomplete-tasks
 var completedTasksHolder = document.getElementById("completed-tasks"); //complete task 

// Add a New task
  var addTask = function(){
  console.log("hskskdj") 
    //When the Button is pressed
      //Create a list item w tex from new text
       //input chechbox
       //Label
       //input (text)
       //Button Edit
       // Button delet
      // Each elements needs modified and appended
 }
// Edit an existing Task  
  var editTask = function(){
  //When edit Pressed
  //if the class of the parent is edit mode
    //switch from edit mode
    //Label text become the inputs value
     // Else
        //witch to editMode
        //input value becomes labels text

      //Toggle editmode on parent
  }
// Delete existing task
  var deleteTask= function(){
  //When the button is presed 
    // Remove the parent list item from ul
}

// mark a tesk as complete
  var taskCompleted = function(){
    console.log("1")
  //when the checkbox is checked 
    // Append list item to the completed task
  }

// mark a task incomplete
   var taskIncomplete = function(){
         console.log("2")
  //when the checkbox is not checked 
    // Append list item to the incompletecompleted task
   }


//Set the click Handler to add task Function
addButton.onclick = addTask;   


var bindTaskEvents= function(taskListItem, checkboxEventHandler){
console.log("bb");

  //Select the childeren of li
  var checkBox = taskListItem.querySelector("input[type=checbox]");
  var editButton =  taskListItem.querySelector("button.edit");
  var deleteButton=  taskListItem.querySelector("button.delete");
  //bind edit task
  editButton.onclick = editTask;
  //bind delete
  deleteButton.onclick =deleteTask;
  //bind checbox event
 checkBox.onchange= checkboxEventHandler;
}
   //cycleover incomplletecomplete tak ul 
    for(var i=0; i < incompleteTaskHolder.children.length; i++) {
// bind events to list items children( taskCompleted)
bindTaskEvents(incompleteTaskHolder.children[i], taskCompleted);
    }


  //cycleover complletecomplete tak ul 
      for(var i=0; i < completedTasksHolder.children.length; i++) {
// bind events to list items children( taskCompleted)
bindTaskEvents(completedTasksHolder.children[i], taskIncomplete);
    }