Hi,
First, I want to comment something. You are using firebird db as I can see, so remove SQL2008 tag from question.
Second, You set your data reader to null and then you are using it... This is obviously "Object reference not set to an instance of an object" exception...
Solution...
Use FbCommand ExecuteReader method and then use reader to read returned records...
string ConnectionString = "Your conn string";
FbConnection addDetailsConnection = new FbConnection(ConnectionString);
addDetailsConnection.Open();
FbCommand fbCommand = new FbCommand("Your sql command string here", addDetailsConnection);
FbDataReader fbDataReader = fbCommand.ExecuteReader();
fbDataReader.Close();
addDetailsConnection.Close();