In ASP.NET AJAX environment, the ASP.NET AJAX controls (including RadControls for ASP.NET AJAX) are created after the page has been loaded. That's why you are not getting client side object of RadTextBox.
All ASP.NET AJAX controls however (including ours), are registered on the page on a later stage - in Sys.Application.Load - you can easily verify that by putting ASP.NET AJAX controls on a page and examining the HTML dump once the page is loaded in the browser. Basically, what happens is that your code is outputted and executed on the page, before the Ajax controls are rendered, which causes the error. Try
the following code to eliminate the error.
sender, EventArgs e)
Another approach is that you can add the client side object of RadTextBox when page loads(OnLoad) and inside that you can access the client side object for each control. Try the following approach.