I keep getting this error in javascript? (radio, checking them in javascript)

Question

For some reason I keep getting this error, I don't know why.

Uncaught TypeError: Cannot set property 'checked' of null js.js:4
check js.js:4
(anonymous function)

Here is my code:

document.body.onload="check()";
function check()
{
document.getElementById("urlchoice").checked=true;
}

Answer 1

First check if document.getElementById("urlchoice") is null or not.

    window.onload = check;

    function check(){
        if(document.getElementById("urlchoice")!=null){ // available
           document.getElementById("urlchoice").checked = true; 
        }
    }