c#更新数据库

我使用Oledb作为我的数据库，我试图在我的数据库中更新"状态"列，每当用户敲入和敲出时。所以它是这样的…

当用户点击时，它将查询数据库的"Status"列之前是"IN"，"OUT"还是空白。当查询为"IN"时，当前状态将设置为"OUT"，反之亦然。更新都是完全按照我想要的方式工作，除了警告"NullReferenceException未处理"，每次我运行它时都出现在这行，我不能继续进一步运行:str = cmd1.ExecuteScalar().ToString();

我对c#非常陌生，如果有人能向我详细解释如何解决这个问题就太好了。谢谢你！

        // prepare command string
        string selectString = @"SELECT Status FROM jiahe where [Tag ID] = '" + textBox1.Text + "'";
        // 1. Instantiate a new command with command text only
        OleDbCommand cmd = new OleDbCommand(selectString, objConnection);
        // 2. Set the Connection property
        cmd.Connection.Open();
        // 3. Call ExecuteNonQuery to send command
        string str = cmd.ExecuteScalar().ToString();
        cmd.Connection.Close();
        if (str.Length == 2 || str.Length == 0)
        {
            //MessageBox.Show("Status: " + str);
            // prepare command string
            string updateString = @"update jiahe set Status = 'OUT' where [Tag ID] = '" + textBox1.Text + "'";
            // 1. Instantiate a new command with command text only
            OleDbCommand cmd1 = new OleDbCommand(updateString, objConnection);
            // 2. Set the Connection property
            cmd1.Connection.Open();
            // 3. Call ExecuteNonQuery to send command
            str = cmd1.ExecuteScalar().ToString();
            MessageBox.Show("Status: OUT");
            cmd1.Connection.Close();

        }
        else
        {
            //string str1 = cmd2.ExecuteScalar().ToString();
            //MessageBox.Show("Status: " + str);
            // prepare command string
            string updateString = @"update jiahe set Status = 'IN' where [Tag ID] = '" + textBox1.Text + "'";
            // 1. Instantiate a new command with command text only
            OleDbCommand cmd1 = new OleDbCommand(updateString, objConnection);
            // 2. Set the Connection property
            cmd1.Connection.Open();
            // 3. Call ExecuteNonQuery to send command
            //string str1 = cmd2.ExecuteScalar().ToString();
            str = cmd1.ExecuteScalar().ToString();
            MessageBox.Show("Status: IN");
            cmd1.Connection.Close();
        }
    }

// 3. Call ExecuteNonQuery to send command
str = cmd1.ExecuteScalar().ToString();

// 3. Call ExecuteNonQuery to send command
str = cmd1.ExecuteNonQuery().ToString();