I am trying to display the seach with pagination by using standard set controller but iam getting below error ,please help where i am going wrong and is the approch is correct?
System.NullPointerException: Attempt to de-reference a null object 
Class.Pagnationwithsetcontroller.getOpportunities: line 50, column 1

VF Page
----------------
<apex:page controller="Pagnationwithsetcontroller">
   <apex:form >
    <apex:pageBlock id="a" >
     <apex:pageBlockSection >
      <apex:inputText value="{!InputString}" title="Search" label="search" >
       <apex:actionSupport action="{!lstofopportunity}" event="onchange" reRender="a" status="status"/> 
      </apex:inputText>
     </apex:pageBlockSection>
     
     <apex:pageBlockSection columns="1">
      <apex:actionStatus id="status" startText="requesting..."/>
       <apex:pageBlockTable value="{!Opportunities}" var="a">
        <apex:column value="{!a.name}"/>
        </apex:pageBlockTable>
        <apex:panelGrid columns="2">
                <apex:commandLink value="first" action="{!first}"/>
                <apex:commandLink value="first" action="{!last}"/>
        </apex:panelGrid>
     </apex:pageBlockSection>
    </apex:pageBlock>
   </apex:form>
</apex:page>

Controller:
----------------
public with sharing class Pagnationwithsetcontroller {
  Public string giveninputString;
  public integer size=2;
  //takes the letter which we type in the input text
  public void setInputString(string s){
   giveninputString=s;
   system.debug('hiiii1 SET'+giveninputString);
  }
  //The "get" method is used to pass data from your Apex code to your Visualforce page
  public string getInputString(){
    system.debug('hiiii2 GET'+giveninputString);
    return null;
  }
  
  public List<opportunity> oppresults{set;get;}
  
  public List<opportunity> getoppresults() {
    system.debug('hiiii oppresults in getoppresults '+oppresults);
    return oppresults;
  }
  
  public  pageReference lstofopportunity(){
    oppresults=(list<opportunity>)[find :giveninputString in all Fields returning opportunity(name)][0] ;
    system.debug('hiiii in lstofopportunity'+oppresults);
    //con= new ApexPages.StandardSetController(oppresults); 
                                                     //system.debug('aaaa11111111111'+con);

    //con.setPageSize(size);
    return null;
  }
 
public ApexPages.StandardSetController con{get; set;} 

Apexpages.standardSetController setcon{get 
                                          {

                                           if(setcon==null){
                                                 system.debug('asassasaasaas'+oppresults);
                                             setcon = new Apexpages.standardsetController(oppresults);
                                                 system.debug('aaaa11111111111'+setcon );

                                             setcon.setPageSize(size);
                                           }return setcon;
                                          }
                                        set;
                                      } 
                                       
public List<Opportunity> getOpportunities() {
               
         return (list<opportunity>)Con.getRecords();
    } 

public void first(){
con.first();
}  
public void last(){
con.last();
} 
  
}