skins2: delete the callback only if it was really added.

modules/gui/skins2/src/dialogs.cpp

@@ -115,11 +115,11 @@ Dialogs::~Dialogs()
         // Detach the dialogs provider from its parent interface
         module_unneed( m_pProvider, m_pModule );
         vlc_object_release( m_pProvider );
-    }
 
         /* Unregister callbacks */
         var_DelCallback( getIntf()->p_libvlc, "intf-popupmenu",
                          PopupMenuCB, this );
+    }
 }