I have two fields Opportunity(Lookup) and Products (Picklist).  On selection of opportunity i had written one soql query such that it retrieves only the products associated with opportunity.  But it is not working.

<apex:actionRegion >
                        <apex:inputField value="{!lineitem.OpportunityId}" >
                        <apex:actionsupport event="onChange" action="{!getprods}" reRender="thePanel"/>
</apex:actionRegion>
<apex:selectList value="{!lineitem.custom__c}" size="1" id="thePanel">
                            <apex:selectOptions value="{!prods}">
                        </apex:selectOptions>
                        </apex:selectList>

and my controller is 

public List<selectOption> getprods()
     {
            List<selectOption> options = new List<selectOption>();
            opp = [select PriceBook2Id from Opportunity where Id=:lineitem.OpportunityId];
            System.debug('sample'+opp);
            for (PriceBookEntry product : [SELECT Name from PriceBookEntry where PriceBook2Id=:opp.PriceBook2Id])
            {
                options.add(new selectOption(product.Name, product.Name));
            }
                return options;
  }

For an existing product it is working.  But for a newly created record the page is not even opening and throwing an error "List has no rows for assignment to SObject"