this._form is null javascript error

Here is one scenario which I had faced in my application for this issue. 

I had included some of the external java script files in my application with below syntax. 

<script src="Common.js" type="text/javascript" /> Wrong 

It is wrong the script tag should be closed with “</script>” instead of "/>". Then only all the scripts will be loaded into the page properly. 

<script src="Common.js" type="text/javascript"></script> Correct