<apex:page standardcontroller="MPI_Category__c"  extensions="MPIController1" action="{!autoRun}"
  tabStyle="Contact">
  <style type="text/css">
  .m1{

   width: 60px;

}
</style>
   <apex:form >
   <apex:actionFunction name="rerenderSg1" rerender="sg1sSelectList1"  >
          <apex:param name="firstParam" assignTo="{!catalog}" value="" />
        
      </apex:actionFunction>
     
      <apex:sectionHeader title="MPI {!mpi.name}"  subtitle="MPI Item Selection" />
<apex:pageBlock >
   <table><tbody>
      <tr>
        <th>Catalog</th>
          <td>
             <apex:selectList id="catalog" styleclass="std" size="1"
                value="{!catalog}" onchange="rerenderSg1(this.value)" >
                    <apex:selectOptions value="{!catalogsSelectList}"/>
             </apex:selectList>
          </td>
          <th>Segment1</th>
          <td>
         
            <apex:selectList id="sg1sSelectList1" styleclass="std" size="1"
                 value="{!sg1}" >
                <apex:selectOptions value="{!sg1sSelectList1}"/>  
            </apex:selectList>
          
          </td>
    
     
      </tr>
     </tbody>
     </table>
       </apex:pageblock>
   </apex:form>
</apex:page>

public with sharing class MPIController1 {
public MPI_Category__c mpi {get;set;}
public MPI_Category__c mpiObject {get;set;}
public MPIProducts__c mpi1 {get;set;}
public Boolean sg4Bool {get;set;}
public Boolean sg3Bool {get;set;}
public Boolean sg2Bool {get;set;}
public Boolean sg1Bool {get;set;}
ApexPages.StandardController controller;
Public Boolean ShowpageBlockFlag{get;set;}
    public MPIController1(ApexPages.StandardController controller) {
    this.controller=controller;
     mpi=[Select  m.Name from MPI_Category__c m where m.id= :ApexPages.currentPage().getParameters().get('id')];
     mpiObject = new MPI_Category__c ();
mpiObject = (MPI_Category__c )controller.getRecord();

    }
public PageReference autoRun() {

      
        return null;

    }

    // Variables to store country and state selected by user
  
         public String catalog {get; set;}  
        
         public String sg1 {get;set;}
          public String sg2 {get;set;}
           public String sg3 {get;set;}
            public String sg4 {get;set;}
   
      // Generates catalog dropdown from country settings
        public List<SelectOption> getCatalogsSelectList()
         {
           List<SelectOption> options = new List<SelectOption>();
            options.add(new SelectOption('', '--SelectOne--'));       

           // Find all the catalogs in the custom setting
          List<product2> prd=  [select id, name, MPI_Catalog__c,MPI_Sg1__c,MPI_Sg2__c,MPI_Sg3__c,MPI_Sg4__c from product2 where MPI_Catalog__c!=null];
         
          set<String> s= new Set<String>();
         
         
         
         
        for(Product2 pr:prd)
        {
         s.add(pr.MPI_Catalog__c);
         
        }
       
        List<String> list1= new List<String>();
       
       
        list1.addAll(s);
       list1.sort();
      
           
            for(String s1:list1)
            {
            options.add(new SelectOption(s1, s1));
            }
        return options;
    }
   
    public List<SelectOption> getSg1sSelectList1() {
     List<SelectOption> options = new List<SelectOption>();
     options.add(new SelectOption('', '--SelectOne--'));  
     System.debug('catalog is -----'+catalog);
   
    List<product2> allsg1 =[select  MPI_Catalog__c,MPI_Sg1__c from product2 where MPI_Catalog__c=:catalog];
    Map<String,String> allSgValues= new Map<String,String>();
   
    for(Product2 p:allsg1)
    {
    if(p.MPI_Catalog__c==this.catalog)
    {
    allSgValues.put(p.MPI_Catalog__c,p.MPI_Sg1__c);
    options.add(new SelectOption(p.MPI_Catalog__c, p.MPI_Sg1__c));
    }
    System.debug('allSgValues are-----'+allSgValues);
    }
   
   
       
        // If no states are found, just say not required in the dropdown.
        if (options.size() > 0) {
      
            options.add(0, new SelectOption('', '--Select One--'));
           
        } else {
      
            options.add(new SelectOption('', 'Not Required'));
          
        }
        return options;
    }
   
   
   
    }

this is my page and controller, getSg1sSelectList1() is not renedering properly.so that Argument 1 canot be null cmg. help me on this