function _setSelectionWatcher(){
        if(htVar.nSelectionWatcher){
            clearInterval(htVar.nSelectionWatcher);
            htVar.nSelectionWatcher = null;
        }

        htVar.nSelectionWatcher = setInterval(function(){
            if(document.getSelection().toString().length === 0){
                htElement.welPopButtonOnBlock.hide();
                clearInterval(htVar.nSelectionWatcher);
                htVar.nSelectionWatcher = null;
            }
        }, 50);
    }
Watch whether selection exists after welPopButtonOnBlock has shown.
If no more selection exists, Hide welPopButtonOnBlock and stop to watching.

@private