ExecuteReader? Connection property has not been initialized

I'm trying to do a login form, but when i try to login, I get an exception.

I'm using a separate file called conexaoDB.cs to create the connection with all info about the DB

Outside the button:

ConexaoDB conexao = new ConexaoDB();

Inside the button:

try
{
    conexao.conn();

    string sql = "SELECT * FROM users WHERE users_login = @login AND users_pass = @senha";

    SqlCommand command = new SqlCommand(sql);
    command.Parameters.AddWithValue("@login", login);
    command.Parameters.AddWithValue("@senha", senha);

    SqlDataReader reader = command.ExecuteReader();

    if (reader.HasRows)
    {
        MessageBox.Show("Popup next window = " + login + " // " + senha);
        tryLogin = true;
    }
    else
    {
        MessageBox.Show("Login ou senha inválidos.");
        tryLogin = false;
    }
}
catch (Exception ex)
{
    MessageBox.Show("Catch: " + ex.Message);
}

This is only to learn SQL connection, is not a real database with real logins and passwords

Drop the ConexaoDB stuff, especially if you are trying to keep one connection open at all time. That is bad practice, built-in connection pooling will work much better.

This should work:

try
{
    string sql = "SELECT * FROM users WHERE users_login = @login AND users_pass = @senha";

    using (var connection = new SqlConnection(connectionString)
    {
        connection.Open();
        using (var command = new SqlCommand(sql, connection)
        {
            command.Parameters.AddWithValue("@login", login);
            command.Parameters.AddWithValue("@senha", senha);

            using (var reader = command.ExecuteReader())
            {
                tryLogin = reader.HasRows;
                if (tryLogin)
                {
                    // Extremely bad practice to show password in a messagebox, but I assume it's for test 
                    MessageBox.Show("Popup next window = " + login + " // " + senha);
                }
                else
                {
                    MessageBox.Show("Login ou senha inválidos.");
                }
            }
        }
    }
}
catch (SqlException ex) // Don't catch too broad an exception, be specific
{
    MessageBox.Show("Catch: " + ex.Message);
}

The usings are taking care of closing and disposing and the SqlConnection goes back to the connection pool and will be reused.

Important:
You should not store passwords as clear text in the database. Best practice is to generate a hash, with a salt and store the hashed value in the database. When authenticating the user you compute the hash again on the password input by the user and compare the two hashes. This assures that your password table is useless to an attacker if it gets stolen.