datagridviewcheckboxcolumn problem getting value

Question

I'm sure everyone has had enough of my posts but these datagridviews are giving me so much hassle! I have three of them that are used for data collection to do a filtered search of a database. each one has two columns left is a list of manufactures, suppliers or categories the right is checkbox column and I have the code below that is used to form a string of all the entries on the left who were checked in the right column. but it fails and says "object reference not set to an instance of an object"

//manufacturer filter used as example

                    if (checkBoxManufacturerFilter.Checked == true) //this combobox isn't in the table its just to indicate weather this filter should be applyed or not

                    {

                        if (FirstFilter == false)

                        {

                            FilteredSearchQuery = FilteredSearchQuery + "and ";

                        }

                        string ManufacturerList = string.Empty;

                        foreach (DataGridViewRow row in DGVmanufacturerFilter.Rows)

                        {

                            if ((bool)row.Cells[1].Value == true) 

                            {

                                ManufacturerList = ManufacturerList + "," + (string)row.Cells[0].Value;

                            }

                        }

                        if (ManufacturerList.Length > 0)

                        {

                            if (ManufacturerList.Substring(0, 1) == ",")

                            {

                                ManufacturerList = ManufacturerList.Substring(1);

                            }

                        }

                        ManufacturerList = "(" + ManufacturerList + ")";

                        string ManufacturerFilterClause = "manufacturer in " + ManufacturerList + " ";

                        FilteredSearchQuery = FilteredSearchQuery + ManufacturerFilterClause;

                        if (FirstFilter == true)

                        {

                            FirstFilter = false;

                        }

                    }