nullのプロパティ'onclick'を設定できないエラー

if(null){
    console.log(1);
} else {
    console.log(2);
}//output2
if(undefined){
    console.log(3);
} else {
    console.log(4);
}//output4

var tan = document.getElementById("tan");
var argeeSure = document.getElementById("argeeSure");
var close = document.getElementById("off");
var sure = document.getElementById("sure");
var body = document.getElementsByTagName("body")[0];
// console.log(body)

if(argeeSure)
argeeSure.onclick = function () {
    tan.style.display = "block";
    body.style.height = "100%";
    body.style.overflow = "hidden";
}
if(close)
close.onclick = function () {
    tan.style.display = "none";
    body.style.height = "auto";
    body.style.overflow = "auto";
}
if(sure)
sure.onclick = function () {
    tan.style.display = "none";
    body.style.height = "auto";
    body.style.overflow = "auto";
}

if(argeeSure ! =null && argeeSure ! = undefined){
    argeeSure.onclick = function () {
        tan.style.display = "block";
        body.style.height = "100%";
        body.style.overflow = "hidden";
    }
}