Hi,
I am stuck at Step 3 of Advanced Apex Specialist. Below is my code.  I am adding below 3 code samples. I have tried overcoming this multiple this but without any results. Please guide me.
 

public  class Product2Extension {
    public List<ProductWrapper> productsToInsert {get;set;}
    public Product2Extension(ApexPages.StandardController controller){
        productsToInsert = new List<ProductWrapper>();
        addRows();
    }

    public void AddRows(){
        for ( Integer i=0; i<Constants.DEFAULT_ROWS; i++ ){
            ProductWrapper prodWrapper = new ProductWrapper();
            productsToInsert.add( prodWrapper );
        }
    }
    public List<ChartHelper.ChartData> GetInventory(){
        return ChartHelper.GetInventory();
    }

    public PageReference Save(){
        SavePoint sp1 = Database.setSavepoint();
        try {
            Map<Integer, Product2> products = new Map<Integer, Product2>();
			Map<Integer, PriceBookEntry> priceBookEntries = new Map<Integer, PriceBookEntry>();    
            Integer index = 0;
            for(ProductWrapper prdWrapper : productsToInsert) {
                if(String.isNotBlank(prdWrapper.productRecord.Name) && prdWrapper.pricebookEntryRecord.UnitPrice!=null && 
                   prdWrapper.productRecord.Initial_Inventory__c!=null && prdWrapper.productRecord.isActive && 
                   prdWrapper.productRecord.Initial_Inventory__c != 0 && prdWrapper.pricebookEntryRecord.UnitPrice!=0){
                          
                	products.put(index,prdWrapper.productRecord);
                    priceBookEntries.put(index,prdWrapper.pricebookEntryRecord);
                    index ++;
                }
            }
            
            insert products.values();
            
            List<PriceBookEntry> pbList = new List<PriceBookEntry>();
            for(Integer mapIndex : products.keySet()) {
            	PriceBookEntry currentPBEntry = priceBookEntries.get(mapIndex);
                if(products.get(mapIndex).Id!=null) {
                    currentPBEntry.Pricebook2Id = Constants.STANDARD_PRICEBOOK_ID;
                	System.debug('' + products.get(mapIndex).Id);
                	currentPBEntry.Product2Id = products.get(mapIndex).Id;
                	currentPBEntry.IsActive = true;
                    pbList.add(currentPBEntry);
                }
                
            }
            
            insert pbList;

            //If successful clear the list and display an informational message
            apexPages.addMessage(new ApexPages.message(ApexPages.Severity.INFO,pbList.size()+' Inserted'));
            productsToInsert.clear();   //Do not remove
            addRows();  //Do not remove
        } catch (Exception e){
			apexPages.addMessage(new ApexPages.message(ApexPages.Severity.ERROR,Constants.ERROR_MESSAGE));
            Database.rollback(sp1);
        }
        return null;
    }
    
    public List<SelectOption> GetFamilyOptions(){
        List<SelectOption> options = new List<SelectOption>();
        options.add(new SelectOption(Constants.SELECT_ONE,Constants.SELECT_ONE));
        for(Schema.PicklistEntry entry : Constants.PRODUCT_FAMILY){
			options.add(new SelectOption(entry.getLabel(),entry.getValue()));
        }
		return options;
    }
    
    
    public class ProductWrapper{
        public Product2 productRecord {get;set;}
        public PriceBookEntry pricebookEntryRecord {get;set;}
        
        public ProductWrapper() {
            productRecord = new Product2(Initial_Inventory__c =0);
            pricebookEntryRecord = new PricebookEntry(Unitprice=0.0);
        }
    }

}

public without sharing class ChartHelper {

  @AuraEnabled
  public static List<chartData> GetInventory(){
    List<chartData> cht = new List<chartData>();

    for(AggregateResult ar : [SELECT Family, SUM(Quantity_Remaining__c) FROM Product2 WHERE Quantity_Remaining__c > 0 AND IsActive = true GROUP BY Family]) {
      cht.add(new ChartData(String.ValueOf(ar.get('Family')), Integer.ValueOf(ar.get('expr0'))));
    }

    return cht;
  }


  public class ChartData {
    public String name {get; set;}
    public Decimal val {get; set;}

    public ChartData(String name, Decimal val){
      this.name = name;
      this.val = val;
    }
  }

}

<apex:page standardController="Product2" extensions="Product2Extension" >
    <apex:sectionHeader title="New Product" subtitle="Add Inventory" />
    <apex:pageMessages id="pageMessages" />
    <apex:form id="form" >
        <apex:actionRegion >
            <apex:pageBlock title="Existing Inventory" id="chart-section">
                <apex:chart height="350" width="450" data="{!inventory}">
                    <apex:axis type="Numeric" position="bottom" title="Quantity Remaining" grid="true"
                               fields="val" dashSize="2">
                        <apex:chartLabel />
                    </apex:axis>
                    <apex:axis type="Category" position="left" fields="name" title="Product Family">
                        <apex:chartLabel rotate="315"/>
                    </apex:axis>
                    <apex:barSeries orientation="horizontal" axis="left" 
                                    xField="val" yField="name"/>
                    <apex:legend position="right"/>
                </apex:chart>
            </apex:pageBlock>
            <apex:pageBlock title="New Products" >
                <apex:pageBlockButtons location="top">
                    <apex:commandButton action="{!save}" value="Save" reRender="chart-section"/>
                </apex:pageBlockButtons>
                <apex:pageBlockButtons location="bottom">
                    <apex:commandButton action="{!addRows}" value="Add" reRender="pageMessages,orderItemTable"/>
                </apex:pageBlockButtons>
                
                <apex:pageBlockTable value="{!productsToInsert}" var="p" id="orderItemTable" >
                    <apex:column headerValue="{!$ObjectType.Product2.Fields.Name.Label}" >
                        <apex:inputText value="{!p.productRecord.Name}" />
                    </apex:column>
                    <apex:column headerValue="{!$ObjectType.Product2.Fields.Family.Label}" >
                        <apex:selectList value="{!p.productRecord.Family}" size="1" multiselect="false" >
                            <apex:selectOptions value="{!FamilyOptions}"/>
                        </apex:selectList>
                    </apex:column>
                    <apex:column headerValue="{!$ObjectType.Product2.Fields.isActive.Label}" >
                        <apex:inputField value="{!p.productRecord.isActive}" />
                    </apex:column>
                    <apex:column headerValue="{!$ObjectType.PriceBookEntry.Fields.UnitPrice.Label}" >
                        <apex:inputText value="{!p.pricebookEntryRecord.UnitPrice}" />
                    </apex:column>
                    <apex:column headerValue="{!$ObjectType.Product2.Fields.Initial_Inventory__c.Label}" >
                        <apex:inputField value="{!p.productRecord.Initial_Inventory__c}" />
                    </apex:column>
                </apex:pageBlockTable>
            </apex:pageBlock>
        </apex:actionRegion>
    </apex:form>
</apex:page>