button wont save to database it comes to exception error - asp.net

Question

i made a asp.net site with 3 textboxes and 1 dropdown list and a save button evvry time i click it, it give back An exception of type 'System.NullReferenceException' occurred in Bon-Temps.dll but was not handled in user code.
it give this error on de code line
DataRow drow = ds.Tables["OpdrachtGever"].NewRow();
my question is why??
SqlConnection cnn = new SqlConnection();
cnn.ConnectionString = System.Configuration.ConfigurationManager.ConnectionStrings["BT-1ConnectionString"].ConnectionString;
cnn.Open();
SqlCommand cmd = new SqlCommand();
cmd.CommandText = "select * from OpdrachtGever";
cmd.Connection = cnn;
SqlDataAdapter da = new SqlDataAdapter();
da.SelectCommand = cmd;
DataSet ds = new DataSet();
da.Fill(ds, "OpdrachtGever");
SqlCommandBuilder cb = new SqlCommandBuilder(da);
DataRow drow = ds.Tables["OpdrachtGever"].NewRow();
drow["Naam"] = TextBox1.Text;
drow["Adres"] = TextBox2.Text;
drow["PostCode"] = TextBox3.Text;
drow["AantalPersonen"] = DropDownList1.SelectedItem;
ds.Tables["OpdrachtGever"].Rows.Add(drow);
da.Update(ds, "OpdrachtGever");

Answer 1

You have named your table as
da.Fill(ds, " OpdrachtGever ");
^ ^
but then you refer to it with
DataRow drow = ds.Tables["OpdrachtGever"].NewRow();
without any spaces.
There is a rogue space also on this line
ds.Tables["OpdrachtGever "].Rows.Add(drow);
^
It is not a good idea to have these spaces in your table name because they are not meaningfull for us (humans) and easy to forget. But, for the indexer of the table collection, a space makes a difference. So, when you pass the name with the wrong or missing spaces, the indexer is not able to find your table and returns null .