Error sending live update: WindowChannelException(CHANNEL_UNREGISTERED, Channel "subwindow_channel" not accessible (may be unregistered, bidirectional pair, or permission denied))

I am unable to send an invokeMethod over a channel.
Whenever I try, the error from the title shows up.

I've ran this in a new project, this being the only dependency I use.
I have a button that runst he following code:

Future<void> _openOrUpdateSubWindow() async {
    setState(() {
      _counter++;
    });

    final payload = 'Patient Data Update #$_counter';
    final activeWindows = await WindowController.getAll();

    for (final controller in activeWindows) {
      final args = controller.arguments as String? ?? '';
      if (args.startsWith('subwindow')) {
        await controller.show();
        try {
          await _channel.invokeMethod('updateData', payload);
        } catch (e) {
          debugPrint('Error sending live update: $e');
        }
        return;
      }
    }

    final controller = await WindowController.create(
      WindowConfiguration(
        hiddenAtLaunch: false,
        arguments: 'subwindow:$payload',
      ),
    );

    await controller.show();
  }

It just doesn't update, and I get the debugPrint every single time.

Start by reproducing the reported flow in a new project using _openOrUpdateSubWindow, WindowController.getAll/create, show, and _channel.invokeMethod('updateData'). Trace how the subwindow_channel is registered and how the updateData handler is connected; done means the existing subwindow receives the payload without CHANNEL_UNREGISTERED.