Without seeing the code it sounds like you are trying to use the result of a .getValue() when it doesn't have a value.
You should have something like:
if (Xrm.Page.getAttribute("new_yourfield").getValue() != null) {
var x = Xrm.Page.getAttribute("new_yourfield").getValue();
}
This checks for a value before trying to use a value that doesn't exist.
If this is't the case, post some code.