The method 'render' isn't defined for the type PDFViewController

Question

I use flutter_pdfview to generate thumbnail image widget of online pdf file, by the following code, and got error: The method 'render' isn't defined for the type PDFViewController.

import 'dart:typed_data';
import 'package:flutter/material.dart';
import 'package:flutter_pdfview/flutter_pdfview.dart';

class PdfThumbnail extends StatefulWidget {
  final String path;
  final double height;
  final double width;

  PdfThumbnail({required this.path, required this.height, required this.width});

  @override
  _PdfThumbnailState createState() => _PdfThumbnailState();
}

class _PdfThumbnailState extends State<PdfThumbnail> {
  late PDFViewController pdfViewController;
  Uint8List? imageBytes;

  @override
  void initState() {
    super.initState();
    _renderPdf();
  }

  void _renderPdf() async {
    final data = await pdfViewController.render(   //here is the error.
      page: 1,
      x: 0.0,
      y: 0.0,
      width: widget.width,
      height: widget.height,
    );
    setState(() {
      imageBytes = data!;
    });
  }

  @override
  Widget build(BuildContext context) {
    return imageBytes != null
        ? Image.memory(
      imageBytes!,
      width: widget.width,
      height: widget.height,
      fit: BoxFit.cover,
    )
        : Container();
  }
}

I guessed that the method has been changed in the upgraded version, so I read the documentation of PDFViewController class but could not find the similar method.