Hi

help me!!

Page

<apex:page controller="PagingController">
<apex:form id="fid">

<apex:pageBlock title="Paging through Categories of Stuff">

<apex:pageBlockButtons location="top">
<apex:commandButton action="{!process}" value="Process Selected" onclick="confirm('Delete');} " />
<apex:commandButton action="{!cancel}" value="Cancel"/>
</apex:pageBlockButtons>
<apex:pageMessages />

<apex:pageBlockSection title="Category Results - Page #{!pageNumber}" columns="1">
<apex:pageBlockTable value="{!categories}" var="c" >
<apex:column width="25px">
<apex:inputCheckbox value="{!c.checked}"/>
</apex:column>
<apex:column value="{!c.cat.Name}" headerValue="Name"/>
</apex:pageBlockTable>
</apex:pageBlockSection>
</apex:pageBlock>

<apex:panelGrid columns="4">
<apex:commandLink action="{!first}">First</apex:commandlink>
<apex:commandLink action="{!previous}" rendered="{!hasPrevious}">Previous</apex:commandlink>
<apex:commandLink action="{!next}" rendered="{!hasNext}" reRender="fid">Next</apex:commandlink>
<apex:commandLink action="{!last}">Last</apex:commandlink>
</apex:panelGrid>

</apex:form>
</apex:page>

controller

public class PagingController {

List<categoryWrapper> categories {get;set;}

// instantiate the StandardSetController from a query locator
public ApexPages.StandardSetController con {get; set;}
public ApexPages.StandardSetController getcon ()
{
if(con == null) {
con = new ApexPages.StandardSetController(Database.getQueryLocator([Select Id, Name FROM Position__c Order By Name limit 100]));
// sets the number of records in each page set
con.setPageSize(5);
}
return con;

}
// returns a list of wrapper objects for the sObjects in the current page set

public List<categoryWrapper> getCategories() {
getcon();
categories = new List<categoryWrapper>();
for (Position__c category : (List<Position__c>)con.getRecords())
categories.add(new CategoryWrapper(category));
System.debug(categories.size());
return categories;
}

// displays the selected items
public PageReference process() {
List<Position__c> positionList=new List<Position__c>();
for (CategoryWrapper cw : categories) {
if (cw.checked)
{
cw.cat.StatusTest__c=true;
positionList.add(cw.cat);
}

//ApexPages.addMessage(new ApexPages.message(ApexPages.severity.INFO,cw.cat.name));
}
update positionList;
return null;
}

// indicates whether there are more records after the current page set.
public Boolean hasNext {
get {
return con.getHasNext();
}
set;
}

// indicates whether there are more records before the current page set.
public Boolean hasPrevious {
get {
return con.getHasPrevious();
}
set;
}

// returns the page number of the current page set
public Integer pageNumber {
get {
return con.getPageNumber();
}
set;
}

// returns the first page of records
public void first() {
con.first();
}

// returns the last page of records
public void last() {
con.last();
}

// returns the previous page of records
public void previous() {
con.previous();
}

// returns the next page of records
public void next() {
con.next();
}

// returns the PageReference of the original page, if known, or the home page.
public void cancel() {
con.cancel();
}

}

After updating on clik of next i'm getting Visualforce Exception???

Can anyone please explain why it is so???