2 回答求，提示這個是我哪里錯了呀？怎么改？Uncaught TypeError: Cannot set property 'onmouseover' of null

1 回答搞不懂錯在哪里。。報錯是21行 cannot set property“onmouseover”of null...

1 回答沒效果，大神幫看看啥問題，chrome瀏覽器報錯是13行，Uncaught TypeError：Cannot set property 'onmouseover' of null

1 回答錯誤提示Cannot read property 'timer' of undefined求解

1 回答練習(xí)js透明度動畫的時候報了這樣的錯誤：part2-opacity.html?_ijt=p2ei6la3k2jfia9gtb6od1s006:27 Uncaught TypeError: Cannot set property 'onmouseover' of null at window.onload