Chrome extension and javascript click/submit error

I am making a chrome extension and I am stuck with javascript and pressing button on page. I have site with form like this on web(isn't mine so i can't edit it):

<input class="button" name="accept" type="submit" value=LoggIn">

And now I automatically open page with username and password filled but now I want that button will be clicked automatically.

I have code of script:

var tabID = 1;
chrome.tabs.create({index:tabID,url:"https://monos
tudby4frd.hisf.no:8001/",active:false,pinned:true}
,
function(tab) {
    tabID = tab.id;
}); 

sleep(3000);
chrome.tabs.executeScript(null, {code:clickButton()});
sleep(3000);

chrome.tabs.getSelected(null, function(tab) {
    chrome.tabs.remove(tabID);
});



function sleep(ms) {
    var dt = new Date();
    dt.setTime(dt.getTime() + ms);
    while (new Date().getTime() < dt.getTime());
}

function clickButton() {
    document.getElementById("accept").click();
}

And debugger says:

Uncaught TypeError: Cannot call method 'click' of null

Thx for any help!