Dear Deon,
Thanks for prompt response.
Provided syntax works well when the dropdown.props.value is null (that is whenever I select one value in dropdown and clear the content, dropdown.props.value is null and syntax works).
But during the session starup or view switchover, the dropdown.props.value is blank, hence the above syntax didnt solve my problem completely.
However I created a custom value as selected label and used the syntax you provided, it works well and solved my issue.
For custom selectedlabel I followed this recommendation:
Syntax I used as:
self.getSibling("Dropdown").custom.selectedlabel == None:
Thanks.