Fixed the following crash:

1) Start PrusaSlicer

2) Add a box to the plater

3) Open Seam Painting or Paint - on supports gizmo

4) Save project

->crash

src/slic3r/GUI/ProjectDirtyStateManager.cpp

@@ -222,7 +222,7 @@ void ProjectDirtyStateManager::reset_after_save()
                 m_last_save.main = main_active_snapshot->timestamp + 1;
         }
         const UndoRedo::Snapshot* saveable_snapshot = get_last_saveable_snapshot(EStackType::Gizmo, active_stack, m_state.gizmos, m_last_save.main);
-        m_last_save.gizmo = saveable_snapshot->timestamp;
+        m_last_save.gizmo = (saveable_snapshot != nullptr) ? saveable_snapshot->timestamp : 0;
     }
 
     reset_initial_presets();