System.NullPointerException: Attempt to de-reference a null object

this error is occurring ?? Here is my code.Where i mistook ?

public PageReference find() 
{         
    for(Account c : [SELECT BillingCountry, Industry, (SELECT Id, Name,Amount , StageName FROM Opportunities) FROM Account WHERE BillingCountry = 'USA'])
    {
            ResultWrapper result = new ResultWrapper(c,c.Opportunities);
            lstResultWrapper.add(result);                
            system.debug('=============lstResultWrapper================='+lstResultWrapper);

    }

    return null;
}     

public class ResultWrapper{

    public Account account {get;set;}
    public Opportunity opportunity {get;set;}

    public ResultWrapper(Account acc,Opportunity opt)
    {
        this.account = acc;
        this.opportunity = opt;
    }
}

Thanks in Advance