private void btnApply_Click(object sender, EventArgs e)
        {
            string filter = "";

            if (chkBoxVoorzieningType.Checked)
            {
                filter += string.Format(" AND voorzieningen = '{0}'", cBoxVoorziening.Text);
            }
            if (chkBoxPostcode.Checked && !string.IsNullOrWhiteSpace(txtPostcode.Text))
            {
                filter += string.Format(" AND lastpostcode LIKE '%{0}%'", txtPostcode.Text);
                window.filteredPostcode = txtPostcode.Text;
            }
            else if (chkBoxPostcode.Checked && string.IsNullOrWhiteSpace(txtPostcode.Text))
            {
                MessageBox.Show(this, "Je hebt geen postcode ingevoerd!", "Lege veld!", MessageBoxButtons.OK, MessageBoxIcon.Warning);
            }
            mainWindow.LoadDataGridResults(mainWindow.application1DataGridCurrentWijkID, mainWindow.application1DataGridCurrentWijkName, filter);
            this.Close();
        }