btn.onmouseover=show;开始提示Uncaught TypeError: Cannot set property 'onmouseover' of null怎么解决！

Question

var ul1=document.getElementById("change_ul");
var btn=document.getElementById("btn");

btn.onmouseover=show;
btn.onmouseout=hide;


function show(){
   document.getElementById("change_ul").style.display="block";
};
function hide(){
   ul1.style.display="none";
}