title: Flutter insert ListView inside showGeneralDialog
body: <p>in <code>Flutter</code> i want to show simple <code>ListView</code> inside <code>showGeneralDialog</code> dialog. but i get this error:</p>
  
  <pre><code>RenderBox was not laid out: RenderFittedBox#d8a97 relayoutBoundary=up1
  'package:flutter/src/rendering/box.dart':
  Failed assertion: line 1694 pos 12: 'hasSize'
  
  showGeneralDialog(
      barrierLabel: "Label",
      barrierDismissible: true,
      barrierColor: Colors.black.withOpacity(0.5),
      transitionDuration: const Duration(milliseconds: 700),
      context: context,
      pageBuilder: (context, anim1, anim2) {
        return Align(
          alignment: Alignment.center,
          child: BuildPagesPopupMenu(),
        );
      },
  ),
  
  class BuildPagesPopupMenu extends StatelessWidget {
    final List&lt;MenuItems&gt; _menuItems = [
      MenuItems(menuTitle: 'test1 ', icon: Icons.settings),
      MenuItems(menuTitle: 'test2 ', icon: Icons.list),
    ];
  
    @override
    Widget build(BuildContext context) {
      return Column(
        children: &lt;Widget&gt;[
          Expanded(
            child: ListView.separated(
              itemBuilder: (context, index) {
                return ListTile(
                  title: Text(_menuItems[index].menuTitle),
                  leading: Icon(_menuItems[index].icon),
                );
              },
              separatorBuilder: (context, builder) {
                return const Divider();
              },
              itemCount: _menuItems.length,
            ),
          ),
        ],
      );
    }
  }
  
  class MenuItems {
    final String menuTitle;
    final IconData icon;
  
    MenuItems({@required this.menuTitle, @required this.icon});
  }
  </code></pre>