This error occurs when you read a property or call a method on a null object . That’s because the DOM API returns null for object references that are blank. An object is expected somewhere and wasn’t provided. So, you will get null is not an object error if the DOM elements have not been created before loading the script. In JavaScript , null is not an object; and won’t work. You must provide a proper object in the given situation.
We can resolve this type of issues by adding an event listener that will notify us when the page is ready. Once the addEventListener is fired, the init() method can make use of the DOM elements.
document.addEventListener('readystatechange', function() {
if (document.readyState === "complete") {
init();
}