private void Save_btn_Click(object sender, EventArgs e)
        {
            // get values from textboxes
            string   goal_name = goalname_textBox.ForeColor == Color.Black ? goalname_textBox.Text : "";
            string   amount    = goalamount_textBox.ForeColor == Color.Black ? goalamount_textBox.Text : "";
            int      cat_id    = categories[Categories_comboBox.SelectedItem.ToString()];
            DateTime term      = term_dateTimePicker.Value;

            // verify if field is filled
            if (goal_name.Equals(""))
            {
                ErrorMessenger.EmptyField("Goal name");
                return;
            }

            if (amount.Equals(""))
            {
                ErrorMessenger.EmptyField("Goal amount");
                return;
            }
            double goal_amount = DB_API.UnMoneyfy(amount);

            // add new goal
            if (!Subcategories_comboBox.SelectedItem.ToString().Equals(this.none))
            {
                cat_id = categories[Subcategories_comboBox.SelectedItem.ToString()];
            }
            try
            {
                DB_API.InsertGoal(account_id, cat_id, goal_name, goal_amount, term);
            }
            catch (SqlException ex)
            {
                ErrorMessenger.Exception(ex);
            }

            PopulateGoalsListBox();
        }