title: ListView.builder inside Stack returns Null Check error used on a null value
body: <p>I'm trying to create a list of tasks inside a circle, when the user taps on it, the list is showed. The problem is that the list of tasks is dynamic, I need to use a <code>ListView.builder</code> but it keeps giving out errors.
  I've checked the following:</p>
  <ul>
  <li>If instead of a <code>.builder</code>, I use a <code>ListView</code>, it works fine.</li>
  <li>The <code>itemCount</code> and <code>shrinkWrap</code> have noting to do with it.</li>
  </ul>
  <blockquote>
  <p>════════ Exception caught by rendering library
  ═════════════════════════════════ RenderBox was not laid out:
  RenderViewport#1d072 NEEDS-PAINT
  'package:flutter/src/rendering/box.dart': Failed assertion: line 1785
  pos 12: 'hasSize' The relevant error-causing widget was ListView
  lib/TaskBubble.dart:43
  ════════════════════════════════════════════════════════════════════════════════</p>
  <p>════════ Exception caught by scheduler library
  ═════════════════════════════════ Null check operator used on a null
  value
  ════════════════════════════════════════════════════════════════════════════════</p>
  <p>════════ Exception caught by Flutter framework
  ═════════════════════════════════ Bad state: Future already completed
  ════════════════════════════════════════════════════════════════════════════════</p>
  </blockquote>
  <p>And here is the code</p>
  <pre><code>class _TaskBubbleState extends State&lt;TaskBubble&gt; {
    double yOffset = 175.0;
    double xOffset = 50.0;
  
    bool hide = true;
  
    List&lt;Task&gt; tasks = [
      new Task(name: &quot;Laundry&quot;, done: false),
      new Task(name: &quot;hide&quot;, done: false)
    ];
  
    @override
    Widget build(BuildContext context) {
      return Positioned(
        top: yOffset,
        left: xOffset,
        child: Draggable&lt;Widget&gt;(
          onDraggableCanceled: (velocity, offset) {
            setState(() {
              yOffset = offset.dy;
              xOffset = offset.dx;
            });
          },
          feedback: FloatingActionButton(
            onPressed: () {},
          ),
          childWhenDragging: Opacity(
            opacity: 0.1,
            child: FloatingActionButton(
              onPressed: () {},
            ),
          ),
          child: Stack(
            children: [
              ListView.builder(
                itemCount: tasks.length,
                shrinkWrap: true ,
                itemBuilder: (context, index) =&gt; ListTile(
                  title: Text(tasks[index].name),
                ),
              ),
              FloatingActionButton(
                onPressed: () {
                  setState(() {
                    hide = !hide;
                  });
                },
              ),
            ],
          ),
        ),
      );
    }
  }
  </code></pre>