Why do I get a null or not an object error?

Two main reasons for this error.
1) A script is loaded before the body content is loaded.
Fix: place the script in a function and call it on a event or (not recommended) place the script below the
body content just above the closing </body> tag
1) You are referencing a parent to child object in the wrong sequence.
Fix:
Use this reference to navigate through the objects and their child objects on a page.
This is a common error in many scripts.
For example
document.radio name is an error
document.form name.radio name is valid

window
|
+-------parent, frames, self, top
|
+-------location
|
+-------history
|
+-------document
|
+--forms
| |
| elements (text fields, textarea, checkbox, password
| radio, select, button, submit, reset)
+--links
|
+--anchors

also on a last note: the need for window.document or window.location has been far long gone.
Unless your viewer is running the
first version of NN or IE it is not needed.