Cannot attach a labeling to 2nd mesh loaded in LabelingViewerApp

[sebmestrallet]

Steps to reproduce

1. Launch labeling_viewer GUI
2. Load a 3D triangle mesh
3. Load or compute a labeling
4. Load a 3D triangle mesh (you can even load the same mesh)
5. Load or compute a labeling

    | o-[Assert      ] Error: Assertion failed: observers_.find(observer) == observers_.end().                                                                      |
    |                  File: /home/catB/sm266019/code/automatic_polycube/ext/geogram/src/lib/geogram/basic/attributes.cpp,                                          |
    |                  Line: 114

Origin

I think it was introduced in this big commit, where include/gui_lableling.h LabelingViewerApp got a Attribute<index_t> labeling_ member variable, to avoid having every labeling-related function to instantiate a Attribute<index_t> from the labeling attribute name.

I did not quite understand how to properly bind/unbind attributes to meshes.