Fix crashes (sometimes) when closing a project, due to overlays

src/Project.cpp

@@ -1124,6 +1124,10 @@ AudacityProject::~AudacityProject()
                      wxCommandEventHandler(AudacityProject::OnCapture),
                      NULL,
                      this);
+
+   // Do this before destroying the overlays
+   delete mRuler;
+   mRuler = nullptr;
 }
 
 AudioIOStartStreamOptions AudacityProject::GetDefaultPlayOptions()