code: public static @Nonnull <T> ExtensionList<T> lookup(Class<T> type) {
          Jenkins j = Jenkins.getInstanceOrNull();
          return j == null ? create((Jenkins) null, type) : j.getExtensionList(type);
      }
docs: Gets the extension list for a given type.
  Normally calls {@link Jenkins#getExtensionList(Class)} but falls back to an empty list
  in case {@link Jenkins#getInstanceOrNull()} is null.
  Thus it is useful to call from {@code all()} methods which need to behave gracefully during startup or shutdown.
  @param type the extension point type
  @return some list
  @since 1.572