MicrosoftAjax.js: Cannot read property '_notified' of null

The problem is that the browser detection code in MicrosoftAjax.js does not detect WebKit (gasp!). Just include this javascript in your site and that should take care of it.

Sys.Browser.WebKit = {};
if (navigator.userAgent.indexOf('WebKit/') > -1) {
Sys.Browser.agent = Sys.Browser.WebKit;
Sys.Browser.version = parseFloat(navigator.userAgent.match(/WebKit/(\d+(.\d+)?)/)[1]);
Sys.Browser.name = 'WebKit';
}

https://forums.asp.net/t/1348484.aspx?error+in+MicrosoftAjax+js+Cannot+read+property+_notified+of+null