CMS/Tips - How to avoid the error message “’null’ is null or not an object”

'Null' error on page in CMS

Sometimes when you load a page, you get a box with the error message shown below. 

or else you see a yellow triangle and "Done, but with errors on page" in the status bar at the bottom of the screen.
 

This happens when you have 'hidden' a page but have asked for a left navigation bar to be included on the page. As the page is hidden, there is no entry to go in the left navigation and the CMS is confused!

So if you hide a page, don't ask for it to have a left navigation bar and this error will go away.