XAMARIN-呈现控件时发生异常

Question

我正在为iOS和Android写作。我想运行设计预览屏幕，但是此错误即将出现。当我想打开一个新项目并查看设计屏幕时，它会起作用。

<?xml version="1.0" encoding="utf-8" ?>

<StackLayout>

    <Entry x:Name="entry_name" Text="deneme1" Placeholder="Ad"></Entry>
    <Entry x:Name ="entry_lastname" Text="deneme2" Placeholder="Soyad"></Entry>
    <Button x:Name="btn_add" Clicked="btn_add_Clicked" Text="EKLE"></Button>
    <ListView x:Name="EmployeeView">
        <ListView.ItemTemplate>
            <DataTemplate>
                <ViewCell>
                    <Grid>
                        <Grid.ColumnDefinitions>
                            <ColumnDefinition Width="0.2*" />
                            <ColumnDefinition Width="0.2*" />
                        </Grid.ColumnDefinitions>
                        <Label Grid.Column="1" Text="{Binding ad}" />
                        <Label Grid.Column="2" Text="{Binding soyad}" />
                    </Grid>
                </ViewCell>
            </DataTemplate>
        </ListView.ItemTemplate>
    </ListView>

</StackLayout>