I have no idea what you mean by "Marks me that I can return a null value at the time of executing it, but the video I was watching for the realization of the project if I let you return that value" or "see the video developer of project, investigation in web page error" - as English sentences they have no meaning. Please use Google Translate if you aren't confident in English.
But ... ExecuteNonQuery returns an integer: you are SELECTing a table of data which may return numerous rows. Quite what you expect this to evaluate to as a boolean value I'm not sure - but I suspect your app will fail with an exception at that point.
Dump the text completely, and just retrieve the rows into your DataTable - it'll be quicker and clearer.
But ... don't do it like that! Never concatenate strings to build a SQL command. It leaves you wide open to accidental or deliberate SQL Injection attack which can destroy your entire database. Always use Parameterized queries instead.
When you concatenate strings, you cause problems because SQL receives commands like:
SELECT * FROM MyTable WHERE StreetAddress = 'Baker's Wood'
The quote the user added terminates the string as far as SQL is concerned and you get problems. But it could be worse. If I come along and type this instead: "x';DROP TABLE MyTable;--" Then SQL receives a very different command:
SELECT * FROM MyTable WHERE StreetAddress = 'x';DROP TABLE MyTable;
Which SQL sees as three separate commands:
SELECT * FROM MyTable WHERE StreetAddress = 'x';
A perfectly valid SELECT
DROP TABLE MyTable;
A perfectly valid "delete the table" command
And everything else is a comment.
So it does: selects any matching rows, deletes the table from the DB, and ignores anything else.
So ALWAYS use parameterized queries! Or be prepared to restore your DB from backup frequently. You do take backups regularly, don't you?