End Google Ads 201810 - BS.net 01 --> i have the following code that appears an error

string query_BranchExposureFactor = "SELECT BranchExposureFactor FROM Questionnaires_Table where Branch_ID = (SELECT Branch_ID FROM [MS Access;DATABASE=" + dialog.FileName + "].Questionnaires_Table1 Where ReferenceYear = '" + txtdateref.Text + "');"; OleDbCommand cmd_BranchExposureFactor = new OleDbCommand(query_BranchExposureFactor, dbConnDest); double branchExposureFactor = ((double)cmd_BranchExposureFactor.ExecuteScalar());
the error is :
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Object reference not set to an instance of an object.