code: public <T extends Extension> T getExtension(Class<T> clazz) {
          if (clazz != null) {
              Extension extension = extensionsMap.get(clazz);
              if (extension != null) {
                  return clazz.cast(extension);
              }
          }
          return null;
      }
docs: Gets the {@code Extension} with the given class.
  
  @param clazz the class of the {@code Extension}
  @return the {@code Extension} or {@code null} if not found/enabled.