public GRNDescriptor getDescriptor(GRN grn) {
        final GRNDescriptorProvider provider = descriptorProviders.get(grn.grnType());
        if (provider == null) {
            throw new IllegalStateException("Missing GRN descriptor provider for GRN type: " + grn.type());
        }
        return provider.get(grn);
    }
@Test
    void getDescriptorWithoutProvider() {
        final GRNDescriptorService service = new GRNDescriptorService(ImmutableMap.of());

        assertThatThrownBy(() -> service.getDescriptor(user))
                .hasMessageContaining(user.type())
                .isInstanceOf(IllegalStateException.class);
    }