This might solve it. Try a different web browser. I assume your using Internet Explorer of some version? If it still does it on a different browser, then that rules out the very likely possibility that this is some error concerning the browser.
Here are some different ones to try:
I'm trying to enter grades in my online grade book program and every time I make an entry I get an error message saying "this.sortImage is null". When I click "enter" it goes away but entering grades takes twice as long. This only happens on my laptop, not on other computers so I know it's not a site problem. Please advise.