空指针异常和空对象

def edit = {
        if (session.user.id != params.id){
            flash.message = \"You can only edit yourself.\"
            redirect(action:list)
            return
            }
        def user = User.get(params.id)
            if (!user){
                flash.message = \"Login first before you edit anyone.\"
                redirect(action:list)
                }
            else{
                return [user:user]
                }
        }

java.lang.NullPointerException: Cannot get property \'id\' on null object

    at com.hyun.UserController$_closure4.doCall(UserController.groovy:25)

    at com.hyun.UserController$_closure4.doCall(UserController.groovy)

    at java.lang.Thread.run(Thread.java:662)

if (!user){
                flash.message = \"Login first before you edit anyone.\"
                redirect(action:list)
                }