code: @Nonnull
    @Override
    public <T> T toObject(@Nonnull Class<T> valueType) {
      T result = super.toObject(valueType);
      Preconditions.checkNotNull(result, "Object in a QueryDocumentSnapshot should be non-null");
      return result;
    }
docs: Returns the contents of the document converted to a POJO.
  
  @param valueType The Java class to create
  @return The contents of the document in an object of type T