为什么我会收到“您不能在空值表达式上调用方法？”

Question

我有一个具有这种结构的 xaml

<Grid>
   <TreeView ItemsSource="{Binding Source={StaticResource xmlData},XPath=//@Name}">
      <TreeView.ItemTemplate>
         <DataTemplate>
            <CheckBox Name="Check1">
               <TreeViewItem Header="{Binding XPath.}"/>
            </CheckBox>
         </DataTemplate>
      </TreeView.ItemTemplate>
   </TreeView>
</Grid>

我正在尝试使用代码添加处理程序

 $var_Check1.addhandler([System.Windows.Controls.Checkbox]::CheckedEvent,$check_checked)

注意：当我从 xaml 中捕获变量时，我将所有变量都设置为 var_，因此存在差异

整个程序背后的想法和想法是让树视图带有复选框，这样我就可以只对选定的项目执行操作。 xml的代码是

<Foods>
    <UnhealthyFoods Name ="Candy"/>
        <Food Foodname = "MMs" >
        </Food>
        <Food Foodname = "Skittles" >
        </Food>
        <Food Foodname = "Starburst" >
        </Food>
    </UnhealthyFoods>
    <HealthyFoods>
        <FoodGroup1 Name ="Vegetables"/>
        <Food Foodname = "Spinach" >
            </Food>
            <Food Foodname = "Brocolli" >
            </Food>
            <Food Foodname = "Greens" >
            </Food>
    </FoodGroup1>
    </HealthyFoods>
    <HealthyFoods>
        <FoodGroup2 Name ="Fruits"/>
        <Food Foodname = "Apples" >
            </Food>
            <Food Foodname = "Pears" >
            </Food>
            <Food Foodname = "Cherries" >
            </Food>
    </FoodGroup2>
    </HealthyFoods>
</Foods>

所以三个复选框应该显示为（假装这些是复选框）

• 糖果
• 蔬菜
• 水果

理想情况下，我单击一个按钮对选定的组执行操作，可惜我做不到那么远