Set a menu item as checked from code

I have an Android application with the following menu item in one of the Activities (which concerns handling a list of names and mac numbers):

<item android:id="@+id/menu_sort_tagg"
      android:icon="@android:drawable/ic_menu_sort_by_size"
      android:title="@string/menu_sort_list" >
      <menu> 
        <group android:checkableBehavior="single">
            <item android:id="@+id/sort_by_name"
                  android:title="@string/sort_by_name" />
            <item android:id="@+id/sort_by_mac"
                          android:title="@string/sort_by_mac" />

     </menu>
</item>

and as the application state changes, I want to be able to pre-check which item in the sort options list that was used last time with the following code:

((MenuItem)findViewById(R.id.sort_by_name)).setChecked(true);

The problem is that this specific line gives me a runtime exception. Does anyone have a clue why?

A look at the log reveals that the runtime exceptions is triggered by a null pointer exception. By changing the code in this way:

MenuItem mi = (MenuItem)findViewById(R.id.sort_by_name);
mi.setChecked(true);

it becomes clear that the exception occurs in the seconds statement, i.e., the MenuItem mi is null. So why fails the first statement to bring a pointer to the correct MenuItem?