Hi, I dont know whats wrong in this code im getting this error msg

Error: Invalid field ischecked for SObject Sample_Item__c

class:

public with sharing class sampleitem {

    public PageReference doSelect() {
        List<sample_Item__c> lstswi = new List<sample_Item__c>();
        for(wrapperSample objws : lstWS){
            if(objws.ischecked == true){
                objws.swi.Item__c = pid;
                update objws.swi;
            }
            
        }
        return (new pagereference('/'+pid).setredirect(true));
    }

    public String pId = apexpages.currentpage().getparameters().get('pid');
    public String wtId = apexpages.currentpage().getparameters().get('wid');
    public List<sample_Item__c> lstSWI{get;set;}
    public wrapperSample objWS{get;set;}
    public List<wrapperSample> lstWS{get;set;}
    public sampleitem(){
        lstSWI = new List<sample_Item__c>();
        lstSWI = new List<sample_Item__c>();
        lstSWI = [select id,name,Stage__r.Name,Item__r.name from sample_Item__c where Template__c =: wtId];
        lstWS = new List<wrapperSample>();
        for(sample_Item__c objSWI : lstSWI){
            objWS = new wrapperSample();
            objWS.swi = objSWI;
            lstWS.add(objWS);
        }
    }
    
    public class wrapperSample{
        public sample_Item__c swi{get;set;}
        public boolean ischecked{get;set;}
    }

 page

<apex:page controller="sampleitem">
      <apex:form >
          <apex:pageblock >
              <apex:pageblocksection >
                  <apex:pageblocktable value="{!lstWS}" var="S">
                      <apex:column headerValue="Select">
                          <apex:inputcheckbox value="{!S.ischecked}"/>
                      </apex:column>
                      <apex:column headerValue="Name">
                          <apex:outputtext value="{!S.swi.Name}"></apex:outputtext>
                      </apex:column>
                       <apex:column headerValue="Stage">
                          <apex:outputtext value="{!S.swi.Stage__r.Name}"></apex:outputtext>
                      </apex:column>
                      <apex:column headerValue="Item Name">
                          <apex:outputtext value="{!S.swi.Item__r.name}"></apex:outputtext>
                      </apex:column>
                  </apex:pageblocktable>
              </apex:pageblocksection>
              <apex:commandButton value="Select" Action="{!doSelect}"/>
          </apex:pageblock>
      </apex:form>