code: @Nonnull
    public T create(@Nonnull DocumentReference documentReference, @Nonnull Object pojo) {
      Object data = CustomClassMapper.convertToPlainJavaTypes(pojo);
      if (!(data instanceof Map)) {
        FirestoreException.invalidState("Can't set a document's data to an array or primitive");
      }
      return performCreate(documentReference, (Map<String, Object>) data);
    }
docs: Creates a new Document at the DocumentReference location. It fails the write if the document
  exists.
  
  @param documentReference The DocumentReference to create.
  @param pojo A map of the fields and values for the document.
  @return The instance for chaining.