Empty string in APEX Validation Expression

Question

I would like to validate removing record from table checking its primary key for integrity constraint violation.

I've created a function returning boolean with the following code:

DECLARE
    n NUMBER;

BEGIN
    SELECT count(*) INTO n
    FROM table_name
    WHERE smth_id = :ID;

    IF n > 0 THEN
        RETURN FALSE;
    ELSE
        RETURN TRUE;
    END IF;
END;

The request is correct, I confirmed it putting real ID instead of :ID, like WHERE smth_id = 105. So it seems to me the problem is in the :ID expression exactly.

Can you advise, how to get the ID of the selected row in function? My code has condition "When Button Pressed - MULTI_ROW_DELETE" and I have ID column on table page.