code: public boolean isExtensionEnabled(String extensionName) {
          if (extensionName == null) {
              throw new IllegalArgumentException("Parameter extensionName must not be null.");
          }
  
          Extension extension = getExtension(extensionName);
          if (extension == null) {
              return false;
          }
          
          return extension.isEnabled();
      }
docs: Tells whether or not an {@code Extension} with the given
  {@code extensionName} is enabled.
  
  @param extensionName the name of the extension
  @return {@code true} if the extension is enabled, {@code false}
  otherwise.
  @throws IllegalArgumentException if the {@code extensionName} is
  {@code null}.
  @see #getExtension(String)
  @see Extension