Please help.. i'm stuck on this. Can't figure out which is wrong with my coding. I received an error '

Any help will be appreciated. Thanks

This is my code:

public class InvoiceController {

public class MissingAccountException extends Exception {}
public class NoLineItemsException extends Exception {}


public Account account {get; private set;}
public Opportunity opportunity {get; private set;}
public Invoice__c order {get; private set;}
public OpportunityLineItem oppLineItem {get; private set;}
public Invoice_Line_Item__c[] invoiceLineItems {get; private set;}
public Invoice_Line_Item__c invoiceLineItem {get; private set;}
public List<Invoice_Line_Item__c> lineItems= new List<Invoice_Line_Item__c>();
private integer itemCount=0;

private PriceBookEntry pbEntry;

public InvoiceController() {

Boolean ShowPartner=false;
Id id = ApexPages.currentPage().getParameters().get('id');
//Id accountId=ApexPages.currentPage().getParameters().get('aId');
Id opportunityId = ApexPages.currentPage().getParameters().get('oId');

opportunity = (opportunityId == null) ? new Opportunity() :
[SELECT name, accountID, CloseDate, Description, StageName, Pricebook2Id FROM Opportunity WHERE id = :opportunityId];

if (opportunity.AccountId != null){
account =new Account();
account= [SELECT name, accountnumber, billingstreet, BillingCity, BillingState, BillingPostalCode, BillingCountry,ShippingStreet, ShippingCity, ShippingState, ShippingPostalCode, ShippingCountry, phone, fax FROM account WHERE id = :opportunity.AccountId] ;
}else {
throw new MissingAccountException('Cannot create an invoice from this opportunity: Please assign an account to the opportunity.');
}

invoice = (id == null) ? new Invoice__c() :
[SELECT Invoice__c.Name FROM Invoice__c WHERE id = :id];

{

//Pre-populate Opportunity and account information
invoice.Name=Opportunity.Name;
invoice.AccountId__c=opportunity.AccountId;
invoice.OpportunityId__c = opportunity.id;
invoice.Status__c='New';

//Retrieve the PriceBook name for Prepopulation in order
string pbID = opportunity.Pricebook2Id;
PriceBook2 pb = (opportunity.Pricebook2Id == null) ? new PriceBook2() :
[Select Id, Name from Pricebook2 where Id=:pbID];
invoice.Pricebook__c=pb.Name;

//Create a new Invoice_Line_Item__c and populate a row for each oppLineItem that exists

integer i = 0;
Invoice_Line_Item = new Invoice_Line_Item__c[0];
for(OpportunityLineItem opp: [SELECT Id, SortOrder, PricebookEntryId, Description, Quantity, ListPrice, UnitPrice, TotalPrice, ServiceDate FROM OpportunityLineItem WHERE Opportunityid = :opportunityId])
{

string pbeID = opp.PricebookEntryId;
pbEntry = (pbeId == null) ? new PriceBookEntry() :
[Select Id, Name, Pricebook2Id, Product2Id, ProductCode from PricebookEntry where Id=:pbeID];


invoiceLineItem = new Invoice_Line_Item__c();
invoiceLineItem.Invoice__c=invoice.Id;
invoiceLineItem.Invoice__c = i+1;
invoiceLineItem.ProductId__c = pbEntry.Product2Id;
invoiceLineItem.Name = pbEntry.Name;
invoiceLineItem.Quantity__c = opp.Quantity;
invoiceLineItem.List_Price__c= opp.ListPrice;
invoiceLineItem.Sales_Price__c=opp.UnitPrice;
invoiceLineItem.Total_Price__c=opp.TotalPrice;
invoiceLineItem.add(Invoice_Line_Item);
i++;
}
itemCount=i;
if (itemCount == 0) {
throw new NoLineItemsException('Unable to create Invoice: Products must be added to the Opportunity before creating an Invoice.');
}
System.Debug('Finished Page Load');

}
public PageReference cancel() {
return (new ApexPages.StandardController(opportunity)).view();
}
//saves invoice and child invoices
public PageReference save() {
System.Debug('Running Save Method');

try
{
insert invoice;
System.Debug(invoice.id);
for(Invoice_Line_Item__c li: invoiceLineItems)
{
li.Invoice__c=invoice.id;
}
insert invoiceLineItems;
//If the Opportunity Stage is not 'Closed Won' then update it
if (Opportunity.StageName != 'Closed Won'){
Opportunity.StageName='Closed Won';
update Opportunity;
}
}catch(System.DMLException e)
{
ApexPages.addMessages(e);
return null;
}

// After Save, navigate to the default view page:
return (new ApexPages.StandardController(invoice)).view();
}

public PageReference loadHelpPage(){
PageReference pg = new PageReference('/apex/CreateInvoiceHelp');
return pg;
}

}