How do i Handle a Case of DBNull so that I exit my application gracefully

I have the  c#  method stated below:

var reader = command.ExecuteReader();
                    if (reader.Read())
                    {
                        if ((string)reader?["formexecutex"] != null)
                        {
                            LoginDetails.staticformexecute = (string)reader["formexecutex"];
                            LoginDetails.staticaddbutton = (string)reader["addbuttonx"];
                            LoginDetails.staticupdatebutton = (string)reader["updatebuttonx"];
                            LoginDetails.staticdeletebutton = (string)reader["deletebuttonx"];

                            results.Add(model);
                        }
                        else
                        {
                            StandardMessages.FormPermission();
                        }
                    }

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During Execution  variable formexecutex can be   "Y" ,  "N"  OR may not even exist  ie is DBNull

How do handle the  DBNull  case to use the else  clause of the If Statement

See  the error i got while trying


Thanks

Olukay