Hi.

This error occurs when I press enter in a textbox, that has AutoCompleteExtender assigned to it and there are not any elements in the auto complete list:

Line: 7244
Error: 'this._completionListElement.childNodes[...].firstChild' is null or not an object

It occurs only the first time I press enter in the textbox: e.g., when I immediately focus the control after the page has loaded and then I press the Entry key.

The simple solution is to initialize the _selectIndex field with -1 instead of null at line 7346 of the "PreviewScript.js" script.

The reason for the problem is that if the initial value is null then condition of the "if" at line 7243 will be satisfied and the next line 7244 will raise the error.