Android Studio：在运行时更改MenuItem图标

I'm trying to add an MenuItem in the Action Bar that when you press it, it changes its icon. However, at the minute when you press it, the app just crashes. I've pasted the relevant parts of the activity's java file but I can't see what I'm doing wrong. When I tested it before I added the bits about changing the icon, it worked fine so I don't think it's the sharedprefs that are making it crash. Can any one help?I'm trying to add an MenuItem in the Action Bar