WPF自定义控件 - 设计器看起来很好，但我遇到了运行时问题

MainWindow.xaml

<Window x:Class="MainWindow"
        xmlns="https://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="https://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:my="clr-namespace:MyStuff;assembly=MyStuff"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
        <TabControl Margin="5">
            <TabItem Header="Start Page" />
            <my:XTabItem Header="Tab 1" Image="Resources/icon1.png" />
        </TabControl>
    </Grid>
</Window>
<Window x:Class="MainWindow"