How can the error "Cannot read property 'classList' of undefined" be resolved in a Bootstrap 5.1 modal popup?

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This error occurs when the JavaScript code tries to access a property or method of an object that is undefined or null. In the case of a Bootstrap 5.1 modal, this error may occur if the element or element ID specified in the JavaScript code is not found or is not valid.

Here are some ways to resolve this error:

1. Check the HTML structure: Make sure the modal element is present in the HTML document and is properly structured with the appropriate classes such as "modal" and "fade". Also, ensure that the modal trigger element has the correct "data-target" attribute that points to the modal ID.

2. Check the JavaScript code: Check the JavaScript code that triggers the modal to ensure it is correct and has the correct syntax. The code should reference the correct modal and trigger elements by their IDs or class names. If using jQuery, ensure that the code is wrapped in a document.ready function.

3. Verify that Bootstrap is properly included: Ensure that the Bootstrap CSS and JavaScript files are included in the HTML document in the correct order. The Bootstrap JavaScript file should be included after jQuery or other JavaScript dependencies.

4. Use console.log to debug: Add console.log statements to the JavaScript code to see if the correct elements are being referenced and to determine where the error is occurring. This can help identify the cause of the error.

5. Use alternative methods: If the error persists, consider using alternative methods to trigger the modal such as using data attributes or jQuery. Alternatively, consider upgrading to a newer version of Bootstrap or seeking help from the Bootstrap community.