Server cant handle Search Results (grails)

I have a List which shows Users

<g:form action="listUsers">
   <g:select id="userListe" name="selectedUser" size="10" onchange="this.form.submit()"
             from="${users.idToShow}"
             value="${selectedUser.idToShow}"/>//edit: idToShow, not itToShow
   </g:form>

I implemented a search function with JQuery, this one works so far

$(function() {
   //When user types, start searching
   $('#userSearch').keyup(function() {
      $.post(search_url, { query: this.value  },
         //data stores the found values from server (works)
         function(data) {

            //first, remove all the values from the userList
            $('#userListe').find('option').remove();

            //split userID and Value, not important
            var userArray = data.split(";");
            for (var i = 0; i < userArray.length; i++) {

               //split userID and Value, not important
               var name = userArray[i].split(':')[0]
               var id = userArray[i].split(':')[1]

               /*
                * This Line solves the problem. Now the Server knows it´s a User
                */ 
                $j('#userListe').append('<option value="'+name+'">'+name+'</option>'
         });
   });
})

The search shows the correct User, but if I click on one (to change values for example), the server doesnt know it's a User, and just shows a null pointer exception.

I know this is kinda interdisciplinary stuff and maybe a bit to large for a support question, but Ill be happy about any small clue.

Thanks a lot, Daniel

method listUsers:

    def listUsers = {

    def foundUsers
    def users

    def sel开发者_如何学编程ectedUser

    foundUsers = User.list()
    users = User.list(fetch: [User: foundUsers.IdToShow])
    users.sort {it.IdToShow}

    selectedUser = users.get(0)

    if (params.selectedUser) {
        selectedUser = User.findByIdToShow(params.selectedUser)
    }

    [users: users, selectedUser: selectedUser]