The problem with this error message is that you can not trace what is the root element that is triggering the exception. If you could see the full stack trace you would have a much better idea if the error is coming from a particular custom widget or if it is coming from a listview or dataview or a button.
In these cases I find it useful to activate the debug option for exceptions. This will then pause javascript execution whenever the program hits an exception and give you the full stack trace. Not only can you see what is the calling element but you can see the variables and their values at this invocation point. This then usually helps me solve the issue.