this._form is null javascript error with Scriptmanager, this problem occur when you are using asp.net ajax extension and another third party js together. i was using jquery when i got this issue.
Solution:i was reffrering the js as following
<script type="text/javascript" language="javascript" src="js/jquery-1.2.6.min.js"
>
but it creates problem when using with asp.net ajax extension
Slightly change it with closing of script
<script type="text/javascript" language="javascript" src="js/jquery-1.2.6.min.js"></script >
or use the best practice syntax and add any third party script as follow.
<asp:scriptmanager id="myScripts" runat="server">
<scripts>
<asp:scriptreference path="js/jquery-1.2.6.min.js">
</asp:scriptreference>
</scripts>
</asp:scriptmanager>
