Xamarin Forms GridView不会将图像放在第二行

Question

我正在尝试在网格顶部并排放置两个按钮，并且在它们下方的图像跨越了两列。但是图像始终停留在两个按钮侧面的顶部，并且不会下移到下一行。

<Grid>
    <Grid.Rowdefinition>
        <RowDefinition Width="Auto"/>
        <RowDefinition Width="Auto"/>
    </Grid.Rowdefinition>

    <Grid.ColumnDefinition>
        <ColumnDefinition Width="Auto"/>
        <ColumnDefinition Width="Auto"/>
    </Grid.ColumnDefinition>

    <telerikInput:RadButton x:Name="signin" Grid.Row="0" Grid.Column="0"        Text="Sign In" BackgroundColor="#2F7DC0" TextColor="white" BorderRadius="0"/>
    <telerikInput:RadButton x:Name="signup" Grid.Row="0" Grid.Column="1" Text="Sign Up" BackgroundColor="#2F7DC0" TextColor="white" BorderRadius="0" />
    <Image Source="https://xamarin.com/content/images/pages/forms/example-app.png" Grid.Row="1" Grid.Column="0" Grid.ColumnSpan="2" />

</Grid>