code: private String getUnavailableServices() {
          StringBuilder missingServices = new StringBuilder();
          if (tokenService.getReference() == null) {
              missingServices.append("tokenService, ");
          }
          if (tokenManager.getReference() == null) {
              missingServices.append("tokenManager, ");
          }
          if (authenticationService.getReference() == null) {
              missingServices.append("authenticationService, ");
          }
          if (authorizationService.getReference() == null) {
              missingServices.append("authorizationService, ");
          }
          if (userRegistry.getReference() == null) {
              missingServices.append("userRegistry, ");
          }
          if (userRegistryService.getReference() == null) {
              missingServices.append("userRegistryService, ");
          }
  
          if (missingServices.length() == 0) {
              return null;
          } else {
              // Return everything but the last ", "
              return missingServices.substring(0, missingServices.length() - 2);
          }
      }
docs: Construct a String that lists all of the missing services.
  This is very useful for debugging.
  
  @return {@code null} if all services are present, or a non-empty
  String if any are missing