Why am I getting "you cannot call a method on a null-valued expression?"

I have an xaml with this structure

<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>

that I am attempting to add a handler to with the code

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

Note: I have all of my variable set to var_ when they are captured from the xaml, hence the discrepancy

The idea and thought behind this entire program is to have the treeview with checkboxes so I can perform an action on only the selected items. The code for the xml is

<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>

So the three checkboxes should show up as (pretend these are checkboxes)

• Candy
• Vegetables
• Fruit

Ideally I click a button that performs an action on the selected groups, alas I can't make it that far