what is Unable to get property 'style' of undefined or null reference error and why am i getting it often

Question

I am creating a game but whenever I try to use the document.getElementById("top_left").style (top_left is an id for a table cell) I get this error "Unable to get property 'style' of undefined or null reference"

Answer 1

onload = function() {
//code
};
//make sure html loads first...

Answer 2

it worked

Answer 3

alright I will try