Fix cleanup of the style GUI utils singleton

Author: wonder-sk

src/app/qgisapp.cpp

@@ -149,6 +149,7 @@
 #include "qgsmapcanvas.h"
 #include "qgsmaplayer.h"
 #include "qgsmaplayerregistry.h"
+#include "qgsmaplayerstyleguiutils.h"
 #include "qgsmapoverviewcanvas.h"
 #include "qgsmaprenderer.h"
 #include "qgsmapsettings.h"
@@ -896,6 +897,8 @@ QgisApp::~QgisApp()
   delete QgsProject::instance();
 
   delete mPythonUtils;
+
+  QgsMapLayerStyleGuiUtils::cleanup();
 }
 
 void QgisApp::dragEnterEvent( QDragEnterEvent *event )