Don't use the [] notation here to test for the existence of a map member,

because this creates an empty entry if it didn't exist. This made the
activation of the dialog mandatory before the next gui subsystem update()
happened. Otherwise fgfs segfaulted.

src/GUI/new_gui.cxx

@@ -219,7 +219,7 @@ NewGUI::newDialog (SGPropertyNode* props)
         return;
     }
     string name = cname;
-    if(!_active_dialogs[name])
+    if(_active_dialogs.find(name) == _active_dialogs.end())
         _dialog_props[name] = props;
 }