Error debugging by null on connection

I want to document a unit test but I'm getting an error:

  

System.Exception: 'Error closing database connection: Object reference not set to an instance of an object.'

My test case has been coded as follows:

using System;
using Microsoft.VisualStudio.TestTools.UnitTesting;
using DAL.Model;
using DAL.Persistence;

namespace GErenciamentoTarefas_TesteUnitario
{
    [TestClass]
    public class UnitTest1
    {
        TarefaDAO td = new TarefaDAO();
        int retorno1 = 0;
        int retorno2 = 0;
        int retorno3 = 0;

        [TestMethod]
        public void testeIncluirTarefa()
        {
            Tarefa tarefa1 = new Tarefa();
            tarefa1.DataEntrega = "19/11/2018";
            tarefa1.Nome = "Nome em Teste";
            tarefa1.Responsavel = "Responsavel em Teste1";
            tarefa1.Tipo = "1";

            Tarefa tarefa2 = new Tarefa();
            tarefa2.DataEntrega = "22/11/2018";
            tarefa2.Nome = "Nome em Teste";
            tarefa2.Responsavel = "Responsavel em Teste2";
            tarefa2.Tipo = "2";

            Tarefa tarefa3 = new Tarefa();
            tarefa3.DataEntrega = "25/11/2018";
            tarefa3.Nome = "Nome em Teste";
            tarefa3.Responsavel = "Responsavel em Teste3";
            tarefa3.Tipo = "3";

            retorno1 = td.Gravar(tarefa1);
            retorno2 = td.Gravar(tarefa2);
            retorno3 = td.Gravar(tarefa3);

            Assert.AreEqual(0, retorno1);
            Assert.AreEqual(0, retorno2);
            Assert.AreEqual(0, retorno3);
        }

    }
}

My connection class is thus encoded

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Data.OleDb;
using System.Data;
using System.Configuration;

namespace DAL.Persistence
{
    public class ConexaoAccess
    {

        protected OleDbConnection Con;
        protected OleDbCommand Cmd;
        protected OleDbDataAdapter Da;

        protected void AbrirConexao()
        {
            try
            {
                //Busca a string de conexão com o banco no arquivo Web.config
                Con = new OleDbConnection(ConfigurationManager.ConnectionStrings["conexaoBanco"].ConnectionString);
                Con.Open();
            }
            catch (Exception ex)
            {
                throw new Exception("Erro ao abrir conexão com banco de dados: " + ex.Message);
            }

        }

        protected void FecharConexao()
        {
            try
            {
                Con = new OleDbConnection(ConfigurationManager.ConnectionStrings["conexaoBanco"].ConnectionString);
                Con.Close();
            }
            catch (Exception ex)
            {
                throw new Exception("Erro ao fechar conexão com banco de dados: " + ex.Message);
            }
        }



    }
}

I tried to fix it by changing the position of the object OleDbConnection :

public class ConexaoAccess
{

protected OleDbConnection Con = new OleDbConnection(ConfigurationManager.ConnectionStrings["conexaoBanco"].ConnectionString);
protected OleDbCommand Cmd;
protected OleDbDataAdapter Da;

but I was not successful.

The debugger points out that the error is always when closing the connection:

What is the solution?