This error is usually caused by using an invalid selector in your code or because you use an undefined selector.
First, make sure your selector is correct, especially when using the ID selector, pay attention to whether the ID name is written incorrectly. You can also use the console to check if the selector is correct, for example using document.querySelector('#xxx')to query the selector.
Second, make sure the element corresponding to your selector exists in the DOM tree. If you are using dynamically generated elements, make sure they are inserted into the DOM before you use selectors.
If you still can't solve the problem, it is recommended to check your code to see if there are any syntax errors or references to undefined variables.