Null-reference-exception-in-xamarin-forms-for-iOS，它在Android中运行良好

Question

XAML课程：

   <?xml version="1.0" encoding="utf-8" ?>
<StackLayout xmlns="http://xamarin.com/schemas/2014/forms" xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" xmlns:local="clr-namespace:XYZ.UI.CustomControls;assembly=XYZ.UI"xmlns:controls="clr- namespace:Xamarin.Forms.Labs.Controls;assembly=Xamarin.Forms.Labs"
         x:Class="XYZ.UI.Views.Layouts.GHeaderLayout">
  <Grid ColumnSpacing="0" BackgroundColor="White" RowSpacing="0">
<Grid.RowDefinitions>
  <RowDefinition Height="80" />
</Grid.RowDefinitions>
<Grid.ColumnDefinitions>
  <ColumnDefinition  Width="*"/>
  <ColumnDefinition Width="50"/>
  <ColumnDefinition Width="2*"/>
</Grid.ColumnDefinitions>
<StackLayout BackgroundColor="White" Grid.Row="0" Grid.Column="0" Orientation="Vertical" Padding="10,0,10,0" >
  <Label Text="{Binding CName}" TextColor="Black" HorizontalOptions="StartAndExpand" VerticalOptions="Start" Font="14" />
  <StackLayout Orientation="Horizontal" HorizontalOptions="FillAndExpand" Spacing="0">
    <Label Text="{Binding JName}" TextColor="Black" HorizontalOptions="Start" VerticalOptions="Start" Font="14"></Label>
    <Image Source="drp_btn.png" WidthRequest="50" HeightRequest="15"  ></Image>
  </StackLayout>
  <StackLayout.GestureRecognizers>
    <TapGestureRecognizer
         Command="{Binding OSCommand}"/>
  </StackLayout.GestureRecognizers>
</StackLayout>

<StackLayout BackgroundColor="White" Grid.Row="0" Grid.Column="1" VerticalOptions="End" Spacing="1">
  <Label x:Name="UserTotalHeader"  Text="Total" TextColor="Black" HorizontalOptions="CenterAndExpand" VerticalOptions="End" Font="14"></Label>
</StackLayout>



<StackLayout x:Name="GEPHeader"   Grid.Row="0" BackgroundColor="White" Grid.Column="2" Orientation="Vertical" HorizontalOptions="FillAndExpand" Spacing="0">
  <Grid RowSpacing="0" BackgroundColor="White">
    <Grid.RowDefinitions>
      <RowDefinition Height="80" />
    </Grid.RowDefinitions>
    <Grid.ColumnDefinitions>
      <ColumnDefinition />

    </Grid.ColumnDefinitions>



    <StackLayout StyleId="A" x:Name="PNameLayout" Grid.Row="0" Grid.Column="0" >


      <ScrollView Orientation="Horizontal" HorizontalOptions="FillAndExpand">
      <local:ItemsSourceBehavior StyleId="C" ItemsSource="{Binding PName}" Orientation="Horizontal" Scroll="Horizontal" >
        <local:ItemsSourceBehavior.ItemTemplate>
          <DataTemplate >
            <StackLayout StyleId="D" Orientation="Vertical" HorizontalOptions="FillAndExpand" Spacing="0" >
             <Label Text="{Binding UD}" LineBreakMode="WordWrap"  XAlign="Center" Font="14"  TextColor="Gray" HeightRequest="40" FontFamily="Bold" VerticalOptions="CenterAndExpand" />
              <Label Text="{Binding UnitCode}" XAlign="Center" Font="14"  TextColor="Gray" HeightRequest="40" FontFamily="Bold" VerticalOptions="CenterAndExpand"/>
            </StackLayout>
          </DataTemplate>
        </local:ItemsSourceBehavior.ItemTemplate>
          </local:ItemsSourceBehavior>
      </ScrollView>
    </StackLayout>

  </Grid>
</StackLayout>

ItemsSourceBehavior：它在IOS中提供NullReference异常（在Android中正常工作）LayoutChildren方法，它继承自Layout。我觉得崩溃发生在调用LayoutChildren方法之前。

namespace XYZ.UI.CustomControls{ 
public class ItemsSourceBehavior : Layout<View>
{

  protected override void LayoutChildren(double x, double y, double width, double height)
    {

        try {
            if (layoutWidth == 0)
            {
                layoutWidth = width;
            }


            double rowHeight = 0;
            double yPos = y, xPos = x;

            foreach (var child in Children.Where(c => c.IsVisible))
            {
                //var request = child.GetSizeRequest(width, height);

                //double childWidth = request.Request.Width;
                //double childHeight = request.Request.Height;
                //rowHeight = Math.Max(rowHeight, childHeight);

                //if (xPos + childWidth > width)
                //{
                //    xPos = x;
                //    yPos += rowHeight + Spacing;
                //    rowHeight = 0;
                if (Children.Count < 4)
                {

                    var region = new Rectangle(xPos, yPos, layoutWidth / Children.Count, 80);
                    LayoutChildIntoBoundingRegion(child, region);
                    xPos += region.Width + Spacing;

                }
                else
                {

                    var region = new Rectangle(xPos, yPos, layoutWidth / 4, 80);
                    LayoutChildIntoBoundingRegion(child, region);
                    xPos += region.Width + Spacing;
                }

            }
        }catch(Exception ex)
        {
            System.Diagnostics.Debug.WriteLine("Exception:ex{0}", ex);
        }}}

输出窗口：

已解决的待处理断点位于＆＃39; ItemsSourceBehavior.cs：362,1＆＃39;取消XYZ.UI.CustomControls.ItemsSourceBehavior.LayoutChildren（双x，双y，双宽，双高）[0x00000]。 线程完成：＃6 线程＆＃39;未知＆＃39; （0x6）已退出代码0（0x0）。 未处理的例外：

System.NullReferenceException：未将对象引用设置为对象的实例

线程完成：＃7 线程＆＃39;未知＆＃39; （0x7）已退出代码0（0x0）。 2016-08-09 19：08：32.542 XYZ [4960：26908] 未处理的异常： System.NullReferenceException：未将对象引用设置为对象的实例   在Xamarin.Forms.StackLayout.LayoutChildren（双x，双y，双宽，双高）[0x00092] in：0   在Xamarin.Forms.Layout.UpdateChildrenLayout（）[0x000c7] in：0   在Xamarin.Forms.Layout.OnSizeAllocated（双倍宽度，双倍高度）[0x0000f] in：0

=============================================== ================== 执行本机代码时获得SIGABRT。这通常表明 单声道运行时或其中一个本机库中的致命错误

由您的应用程序使用。

发射成功。该应用程序＆＃39; XYZ＆＃39;已在iPad 2 iOS 8.4＆＃39;上成功推出。 该应用已被终止。