Hollowing gizmo fix: selection of single holes was recently broken

src/slic3r/GUI/Gizmos/GLGizmoHollow.hpp

@@ -43,6 +43,8 @@ private:
     void hollow_mesh(bool postpone_error_messages = false);
     bool unsaved_changes() const;
 
+    ObjectID m_old_mo_id = -1;
+
     // bool  m_show_supports = true;
     float m_new_hole_radius = 2.f;        // Size of a new hole.
     float m_new_hole_height = 6.f;