Thanks for your answer, but I think something else - not this "clear button". So there is a demo like this:
I attached the screenshot: there is a clear button on the right side of the multiselect object. (see red circle) I thought that this option set the visible of this clear button to true or false. Is it correct? If it is, there is not differnce in the output: It will never appear this button. This is my problem...