Protect calls to QAction 'object_{group,ungroup}'

The "object_group" and "object_ungroup" actions are not created in any code from libs/ directory. They are created in the plugins/ one, so there may be cases where they do not exist. This is not the case in Calligra itself, and the code is not failing in calligra or calligraconverter, but I think that nothing can guarantee that these two actions exist and it's safer to protect their call. Do you agree ?