C# 对实例方法的委托不能为null'；这'；在Windows Phone 8.1自定义控件中

在针对windows phone 8.1的非silverlight项目中，我遇到了问题。我创建了自定义控件PullToRefreshPanel，添加GridView后，Xaml设计器显示错误：

Delegate to an instance method cannot have null 'this'

如何捕捉这个错误？我怎么知道是因为我的错误

控件的类型和样式：

 <Style TargetType="controls:PullToRefreshPanel">
    <Setter Property="FontSize" Value="24" />
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="controls:PullToRefreshPanel">
                <Border Background="{TemplateBinding Background}" BorderBrush="{TemplateBinding BorderBrush}">

                    <VisualStateManager.VisualStateGroups>
                        <VisualStateGroup x:Name="VisualStates">
                            <VisualState x:Name="Normal" />
                            <VisualState x:Name="Pull">
                                <Storyboard>
                                    <FadeInThemeAnimation TargetName="PullContent" />
                                    <FadeOutThemeAnimation TargetName="RefreshContent" />
                                </Storyboard>
                            </VisualState>
                            <VisualState x:Name="Refresh">
                                <Storyboard>
                                    <FadeInThemeAnimation TargetName="RefreshContent" />
                                    <FadeOutThemeAnimation TargetName="PullContent" />
                                </Storyboard>
                            </VisualState>
                        </VisualStateGroup>
                    </VisualStateManager.VisualStateGroups>

                    <ScrollViewer x:Name="ScrollViewer"
                                  HorizontalScrollMode="Disabled"
                                  VerticalScrollBarVisibility="Hidden"
                                  ZoomMode="Disabled">
                        <StackPanel x:Name="StackPanel">
                            <Grid Name="PullGrid" Height="{TemplateBinding InvisiblePullRange}">
                                <Grid Height="{TemplateBinding PullRange}" VerticalAlignment="Bottom">
                                    <ContentControl x:Name="RefreshContent"
                                                    HorizontalAlignment="Center"
                                                    VerticalAlignment="Center"
                                                    Content="{TemplateBinding RefreshContent}" />
                                    <ContentControl x:Name="PullContent"
                                                    HorizontalAlignment="Center"
                                                    VerticalAlignment="Center"
                                                    Content="{TemplateBinding PullContent}" />
                                </Grid>
                            </Grid>
                            <Grid x:Name="ContentGrid">
                                <ContentPresenter />
                            </Grid>
                        </StackPanel>
                    </ScrollViewer>
                </Border>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>

我使用Visual Studio 2015和Windows Phone 8.1 XAML项目。 有什么想法吗

更新：

---

也许是一个bug Xaml设计器？该应用程序运行良好。仅在Xaml设计器中出错。 我从控制代码中删除了所有引用EventHandler PullToRefresh，错误仍然存在。 这就是不断出现的错误：

---

您必须实例化PullToRefresh eventhandler。它正在获取空引用异常。检查此答案如何正确执行？该方法是通过Xaml分配的。还是我不明白的？在我所看到的创建EventHandler的所有示例中，都使用了与我相同的代码。我认为这个错误是由Xaml引起的？也许是Xaml设计器的错误？该应用程序运行良好。仅在Xaml设计器中出错。

 <Style TargetType="controls:PullToRefreshPanel">
    <Setter Property="FontSize" Value="24" />
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="controls:PullToRefreshPanel">
                <Border Background="{TemplateBinding Background}" BorderBrush="{TemplateBinding BorderBrush}">

                    <VisualStateManager.VisualStateGroups>
                        <VisualStateGroup x:Name="VisualStates">
                            <VisualState x:Name="Normal" />
                            <VisualState x:Name="Pull">
                                <Storyboard>
                                    <FadeInThemeAnimation TargetName="PullContent" />
                                    <FadeOutThemeAnimation TargetName="RefreshContent" />
                                </Storyboard>
                            </VisualState>
                            <VisualState x:Name="Refresh">
                                <Storyboard>
                                    <FadeInThemeAnimation TargetName="RefreshContent" />
                                    <FadeOutThemeAnimation TargetName="PullContent" />
                                </Storyboard>
                            </VisualState>
                        </VisualStateGroup>
                    </VisualStateManager.VisualStateGroups>

                    <ScrollViewer x:Name="ScrollViewer"
                                  HorizontalScrollMode="Disabled"
                                  VerticalScrollBarVisibility="Hidden"
                                  ZoomMode="Disabled">
                        <StackPanel x:Name="StackPanel">
                            <Grid Name="PullGrid" Height="{TemplateBinding InvisiblePullRange}">
                                <Grid Height="{TemplateBinding PullRange}" VerticalAlignment="Bottom">
                                    <ContentControl x:Name="RefreshContent"
                                                    HorizontalAlignment="Center"
                                                    VerticalAlignment="Center"
                                                    Content="{TemplateBinding RefreshContent}" />
                                    <ContentControl x:Name="PullContent"
                                                    HorizontalAlignment="Center"
                                                    VerticalAlignment="Center"
                                                    Content="{TemplateBinding PullContent}" />
                                </Grid>
                            </Grid>
                            <Grid x:Name="ContentGrid">
                                <ContentPresenter />
                            </Grid>
                        </StackPanel>
                    </ScrollViewer>
                </Border>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>