some projects cannot be removed
example from pg-sandbox:
2019-11-15 14:58:51,287 ERROR [Thread-20] (SqlExceptionHelper.java:142) - ERROR: update or delete on table "element_table" violates foreign key constraint "structural_state_table_species_fk" on table "structural_state_table"
Detail: Key (id)=(475568) is still referenced from table "structural_state_table".