Hey there karthik,
I've run into this annoying bug as well. What helped me was to delete the "null" from the generated tags and then inserting the URL field from the right side as in this step:

but between the tags (where the "null" used to be), so your result would be:
<hyperlink><name>Kindly Click here</name><value>{Record URL(Dynamic)(Opportunity)}</value></hyperlink>
Have you tried this yet?