Forms :: Stop Saving When Detail Record Is Null?

I have master detail block just consider Forms A as a master and Forms B as a Detail,when my cursor is in form A i try to save the record, it is saved Form A and putting Form B data as null, i have to put a validation to stop this activity, i have to inform the user "Provide values for form B text item",

I created a post block trigger in form A

Begin

If (:FORM-B.TEXT1 is null)l or( :FORM-B.TEXT2 is null)
then
Message( 'provide value for FORM-B');
Message( 'provide value for FORM-B');
raise form_trigger_failure;
ELSE
NULL;
END IF
END;

Im Getting error message and navigation got stopped in last item of form A itself,i need to show error message and cursor control should move to first item of form -B which trigger i have to use for this.