After two hours, I decided to finally ask all of you. Within 5 minutes I found that wrapping it with Flexible eliminates the error.

Here is the code:

  Widget build(BuildContext context) {
    return Flexible(
      child: ListView(
        padding: const EdgeInsets.symmetric(),
        children: tasks,
      ),
    );
  }

Md. Yeasin Sheikh also suggested just a minute later: wrapping it with Expanded and that works too. Not sure what I missed in the documentation about ListView.