Visualforce Page:
<apex:page controller="AccountRelatedContactsController">
<apex:form id="Accform">
<apex:pageMessages id="msg"></apex:pageMessages>
<apex:pageblock title="Account Related Contacts">
<apex:pageblockSection >
<apex:selectList label="Select Account Name" size="1" value="{!selectedAccount}">
<apex:actionSupport event="onchange" action="{!LoadContacts}"  rerender="Accform" status="actStatusId"/>
<apex:selectoptions value="{!ContactsInOptionslist}">
</apex:selectoptions>
</apex:selectList>
</apex:pageblockSection>
<apex:pageblockSection id="ContactsListPB" columns="1">
<apex:pageblockTable value="{!listOfContacts}" var="con">
<apex:column headerValue="First Name">
                    <apex:inputField value="{!con.FirstName}"> 
                       <!-- <apex:inlineEditSupport event="ondblclick" showOnedit="update"/> -->
                    </apex:inputField>
                </apex:column>
                <apex:column headerValue="Last Name">
                    <apex:inputField value="{!con.lastName}"> 
                       <!-- <apex:inlineEditSupport event="ondblclick" showOnedit="update"/> -->
                    </apex:inputField>
                </apex:column>
                <apex:column value="{!con.Phone}"/>
<apex:column >
<apex:commandLink value="Delete" action="{!deleteRecord}" onclick="if(!confirm('Are you sure?')){return false;}" rerender="Accform">
<apex:param name="ContactToDelete" value="{!con.id}" assignTo="{!RecordToDelete}"/>
</apex:commandLink>
</apex:column>
</apex:pageblockTable>
</apex:pageblockSection>
<apex:pageBlockButtons >
<apex:actionStatus id="actStatusId" >
    <apex:facet name="start" >
      <img src="/img/loading.gif" />                    
    </apex:facet>
</apex:actionStatus>
<apex:commandbutton value="Add" action="{!Addrow}" rerender="Accform" status="actStatusId" disabled="{!selectedAccount==null}"/>
<apex:commandButton value="Save" action="{!savecontact}" rerender="Accform" status="actStatusId" disabled="{!selectedAccount==null}"/>
</apex:pageBlockButtons>
<apex:pageblockButtons location="bottom">  
            <apex:commandButton value="<<"   action="{!beginning}" disabled="{!prev}" rerender="Accform"/>  
            <apex:commandButton value="<"  action="{!pre}" disabled="{!prev}" rerender="Accform" />  
            <apex:commandButton value=">" action="{!next}"  disabled="{!nxt}" rerender="Accform"/> 
             <apex:commandButton value=">>"  action="{!end}" disabled="{!nxt}" rerender="Accform" />                    
        </apex:pageblockButtons> 
</apex:pageblock>

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



apex class:
public class AccountRelatedContactsController{
    public List<SelectOption> ContactsInOptionslist{get;set;}
    public List<Contact> listOfContacts{get;set;}
    public string selectedAccount{get;set;}
    public Id RecordToDelete {get; set;}
    private integer numrec = 0;
    private integer index = 0;
    private integer blocksize = 2;
    public List<Contact> cont = new List<Contact>();
    public List<contact> cclist{set;get;}
    public AccountRelatedContactsController(){
        Id id = ApexPages.currentPage().getParameters().get('id');
         numrec = [SELECT count() FROM contact];
        system.debug('numrec---->'+ numrec);
        List<Account> listOfAccounts = [SELECT Id,Name FROM account ORDER BY Name];
        ContactsInOptionslist = new List<SelectOption>();
        listOfContacts = new List<Contact>();
        if(!listOfAccounts.isEmpty()){
            ContactsInOptionslist.add(new SelectOption('','--None--'));
            for(Account acc:listOfAccounts ){
             ContactsInOptionslist.add(new SelectOption(acc.Id,acc.Name));
             
            }
            
        }
        
    }
   public PageReference LoadContacts(){
        if(selectedAccount != null){
        listOfContacts =[select id, firstname, lastname,phone from contact where accountid =:selectedAccount LIMIT :blocksize OFFSET :index];
       
        }
         return null;
    }
    public pagereference deleteRecord(){
        system.debug('RecordToDelete-->'+RecordToDelete);
        DELETE new contact(Id = RecordToDelete);
        ApexPages.addmessage(new ApexPages.message(ApexPages.severity.INFO, 'Contacts Deleted Successfully..!'));
        LoadContacts();
        return null;
    }
    
     public pagereference  savecontact() {
     upsert listOfContacts ;
     ApexPages.addmessage(new ApexPages.message(ApexPages.severity.INFO, 'Contacts Updated Successfully..!'));
        return null;
    
     
    }
    public pagereference Addrow()
    {
        listOfContacts.add(new contact());
        return null;
    }  
    public pagereference beginning(){
        index = 0;
        LoadContacts();
        return null;
    }
    public pagereference next(){
        if((index + blockSize) < numrec){
           index = index + blocksize;
           }
       LoadContacts();
       return null;
        
    }
    public pagereference  pre(){
        if(index >= blocksize){
        index = index - blocksize;
        
        }
        LoadContacts();
        return null;
        
    }
            
     Public pagereference end()
    {
        index= numrec-math.mod(numrec,blocksize);  
         LoadContacts();
         return null;
    }
       public boolean getprev(){
      if(index == 0 || selectedAccount==null){
      return true;
      }
     else {
    return false;
    }
    }
    public boolean getnxt()
    {
    if((index + blocksize) > numrec || selectedAccount==null ){
    
    return true;
    }
    else {
    
    return false;
    }
    }
    
    }

Can someone please tell me what im doing wrong?