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";}