docs: protected final Injector getInjector() {
          if (injector != null) {
              throw new IllegalStateException("Injector already created.");
          }
          try {
              injector = createInjector();
          }
          catch (Exception e) {
              LOG.error("Failed to created injector", e);
              throw new ProvisionException("Failed to create injector", e);
          }
          return injector;
      }