fix crash caused by use bar instance after it is freed (use-after-free)

include/bar.hpp

@@ -54,7 +54,7 @@ class BarIpcClient;
 }
 #endif  // HAVE_SWAY
 
-class Bar {
+class Bar : public sigc::trackable {
  public:
   using bar_mode_map = std::map<std::string, struct bar_mode>;
   static const bar_mode_map PRESET_MODES;