How to make sure the return is executed only after the build callback is completed?

While debugging this code snippet, I see that the the build callback is executed after the return, hence leaving the late w not assigned.

How can I do to schedule the call properly?

import 'package:pdf/widgets.dart' as pw;



class PdfPageIdentification {
  PackageInfo packageInfo;
  String smartboxId;
  String smartboxMacAddress;
  pw.Widget? label;
  pw.Page? page;

  PdfPageIdentification(
      {required this.packageInfo,
      required this.smartboxId,
      required this.smartboxMacAddress,
      this.label,
      this.page});
}


/// Creates the battery identification label as a PDF file
  static Future<PdfPageIdentification> identificationSheetPage(
      PeripheralManufacturerData data) async {
    final packageInfo = await PackageInfo.fromPlatform();

    late PdfPageIdentification w;
    final page = pw.Page(
        pageFormat: PdfPageFormat.a4,
        margin: pw.EdgeInsets.symmetric(horizontal: 180),
        // margin: pw.EdgeInsets.all(40),
        build: (pw.Context context) {
          w = identificationSheet(data, packageInfo, context);

          return w.label!;
        });

    w.page = page;
    return w;
  }