code: @Override
      public MutableDictionary getDictionary(@NonNull String key) {
          return ((MutableDictionary) internalDict).getDictionary(key);
      }
docs: Get a property's value as a Dictionary, which is a mapping object of an dictionary value.
  Returns null if the property doesn't exists, or its value is not an dictionary.
  
  @param key the key.
  @return the Dictionary object or null if the key doesn't exist.