Error in webservice: javax.servlet.Servletexception: java.lang.Nullpointerexception

Question

I am trying to execute a query in my Webservice, but the function that query is inside of, break from nothing.

My code:

public boolean excluir(String usuario)
{
    String sql = "DELETE * FROM listaproduto where uclogin=?";
    Boolean retorno = false;
    PreparedStatement pst = Conexao.getPreparedStatement(sql);
    try {
        pst.setString(1, usuario); //Ele quebra nessa parte.
        if(pst.executeUpdate()>0)
        {
            retorno = true;
        }
    } catch (SQLException ex) {
        Logger.getLogger(ProdutoDAO.class.getName()).log(Level.SEVERE, null, ex);
        retorno = false;
    }
    return retorno;
} 

Error:

GET Requestfailed Requestfailed --> Status: (500) Answer: { HTTP Status 500 - Internal Server Error

type Exception report

messageInternal Server Error

descriptionThe server encountered an Internal error that prevented it from fulfilling this request.

Exception

javax.servlet.Servletexception: java.lang.Nullpointerexception

root cause

java.lang.Nullpointerexception

note The full stack Traces of the Exception and its root causes are available in the Glassfish Server Open Source Edition 4.1.1 logs. Glassfish Server Open Source Edition 4.1.1

}