Cannot read property 'addEventListener' of null

This error occurs when you are trying to add an event listener to an element that doesn't exist or hasn't been loaded yet.

To fix this error, ensure that the element you are trying to add an event listener to exists in your HTML code and that it has been loaded before your JavaScript code runs. You can do this by putting your JavaScript code at the bottom of your HTML file, or by wrapping your JavaScript code in a function that runs after the page has loaded, such as the window.onload function.

Example:

window.onload = function() {
  var myBtn = document.getElementById("myButton");
  myBtn.addEventListener("click", myFunction);
}

function myFunction() {
  console.log("Button clicked!");
}

In this example, the JavaScript code waits for the window to load before finding the "myButton" element and adding a click event listener to it.