[Import] some vtk crash at import in 3.2 and master

[mathildemerle]

I've got an old batch of mha and vtk files. Three of them are crashing at import (nb: code name umich, data LA&AO, LVENDO and LVEPI), the other ones are perfectly imported.

Log error in master branch for these data

0x00007fffce545460 in medVtkView::buildThumbnail (this=0x5555566121d0, size=...)
    at /home/mathildemerle/Dev/medInria4.0/medInria-public/src/plugins/legacy/medVtkView/medVtkView.cpp:466
466	    d->mainWindow->show();
 
0x00007ffff78aa79b in medAbstractView::generateThumbnail (this=0x5555566121d0, size=...)
    at /home/mathildemerle/Dev/medInria4.0/medInria-public/src/layers/legacy/medCoreLegacy/views/medAbstractView.cpp:345
345	    return buildThumbnail(size);

0x00007ffff79621f7 in medAbstractData::generateThumbnailInGuiThread (this=0x7fffc0004140, 
    size=...)
    at /home/mathildemerle/Dev/medInria4.0/medInria-public/src/layers/legacy/medCoreLegacy/data/medAbstractData.cpp:271
271	    return view->generateThumbnail(size);

medInria should at least not crash and show an error message, but it would be nice to successfully open these files.