未捕获的TypeError：无法读取null的属性“value”

I'm getting error in this code, I'm trying to do an event where in when the page is load, it will do the event. But the problem is when I go to other function, but same page, it gets a error of null on that variable. It has no problem when I execute this codes, but when I'm on other part of my codes this error occurs. I'm getting error in this code, I'm trying to d