title: matlab gui popup menu error message
body: <p>I have created a GUI and there is a pop-up menu in it. I want to know how I can generate an error dialogue when the pop-up menu is not open/used by a user. For the edit box, I use "isempty", but that doesn't work for the pop-up menu.
  if the user don't open the pop-up menu and don't select any choice from this pop-up menu i want my program display an error message for remind the user select a choice.</p>
  
  <p>Thanks and I look forward to hearing your solution</p>