Safari And Chrome Not Accepting GetElementsByTagName?

I'm using getElementsByTagName() to retrieve some elements and do something with them. It works fine in FF but not in Chrome and Safari.

In Chrome it says: "Uncaught TypeError: Cannot call method 'getElementsByTagName' of null"

Here's the code:

Code:
function popUpSAPWindow(){
// Find all links in the page and put them into an array.
//Below is the line that gives me trouble
var linksInOrderLinesTable =

[Code]....