iReport Java Subreport - The supplied java.sql.Connection object is null

Hello

I'm studying iReport Design and started using Connection in my fillReport. Until then, everything was fine. The problem is that, now, I started using Collection, through the commands:

List<Vendas> vendas = new VendasDAO().getVendasPorPeriodo(
                new SimpleDateFormat("yyyyMMdd").parse("20180101"),
                new SimpleDateFormat("yyyyMMdd").parse("20180131")
        );

JRDataSource dataSource = new JRBeanCollectionDataSource(vendas);

That way, when I try to run, I get the following error:

  

WARNING: The supplied java.sql.Connection object is null.