core: fixed handling of button callback in popups:

Button use themselve as callback, and this was causing troubles in the removePopUp method.
This patch work around this by considering a Button widget in the same way as None (we
never use Button directly in popups).

fix 183

sat_frontends/primitivus/primitivus

@@ -595,7 +595,9 @@ class PrimitivusApp(QuickApp, InputHistory):
         @param widget(None, urwid.Widget): if not None remove this popup from front or queue
         """
         # TODO: refactor popup management in a cleaner way
-        if widget is not None:
+        # buttons' callback use themselve as first argument, and we never use
+        # a Button directly in a popup, so we consider urwid.Button as None
+        if widget is not None and not isinstance(widget, urwid.Button):
             if isinstance(self.loop.widget, urwid.Overlay):
                 current_popup = self.loop.widget.top_w
                 if not current_popup == widget: