C# Databases SQLite Locked Database Error

I am currently writing a code using C# and SQLite. There is an error being throwing stating that the database is locked twice in a message box.

The query works on SQLite DB Browser however, when it is placed in C# code it throws the error.

Here is the code that is giving me an error:

cmd.CommandText = "UPDATE customers SET Bill = Bill - "+textBox2.Text+" WHERE customers.CustomerID = " + textBox1.Text + ";";

There seems to be an issue with the equals sign, might be something wrong with the arithmetic process.

Complete code:

 SQLiteConnection myconn = new SQLiteConnection(@"Data Source = C:\Users\chick\Newspaper.db");
        SQLiteCommand cmd = myconn.CreateCommand();
        cmd.CommandText = "UPDATE customers SET Bill = (Bill - "+textBox2.Text+") WHERE customers.CustomerID = " + textBox1.Text + ";";
        myconn.Open();
        try
        {
            cmd.ExecuteNonQuery();
            MessageBox.Show("Succesfully Update");
        }
        catch(Exception ex)
        {
            MessageBox.Show(ex.Message);
        }

UPDATE: Changed format to using() {} however it is still not working. Program is crashing

New Code:

   using (SQLiteConnection myconn = new SQLiteConnection(@"Data Source = C:\Users\chick\Newspaper.db"))
        {

            var sql = "Update customers SET Bill = Bill - @pay WHERE customers.CustomerID = @cid;";
            myconn.Open();
            using (var cmd = new SQLiteCommand(sql, myconn))
            {
                cmd.Parameters.AddWithValue("@cid", textBox1.Text);
                cmd.Parameters.AddWithValue("@pay", textBox2.Text);
                cmd.ExecuteNonQuery();
            }
        }