I was puzzled today by the following exception coming out of one of our web application: this._form has no properties. The exception occurred even on simple pages that had a form and a Script Manager tag. After troubleshooting the issue, I traced it down to the script tag on the site master page. Apparently, JavaScript tags cannot be self-closing.  

Replacing 

             <script language="javascript" type="text/javascript" src="my.js"/> 

with  

             <script language="javascript" type="text/javascript" src="my.js"></script>  

resolved the issue. Happy coding!