This is evil: the hash table of "insert_data" is created on demand
when GimpContainerView API is used, using a value_free_func that is
set in the interface_init functions of its implementors. Therefore, no
GimpContainerView API must be called from any init() function, because
the interface_init() function of a subclass that sets the right
value_free_func might not have been called yet, leaving the
insert_data hash table without memory management.

Call GimpContainerView API from GObject::constructed() instead, which
runs after everything is set up correctly.