尝试调用虚方法＆＃39; void android.widget.ListView.setAdapter（android.widget.ListAdapter）＆＃39;在null对象上引用android studio

Question

list = (ListView) findViewById( R.id.simple_list );
    String[] planets = new String[] { "Mercury", "Venus", "Earth", "Mars",
            "Jupiter", "Saturn", "Uranus", "Neptune"};
    ArrayList<String> planetList = new ArrayList<String>();
    planetList.addAll( Arrays.asList(planets) );

    // Create ArrayAdapter using the planet list.
    listAdapter = new ArrayAdapter<String>(getApplicationContext(), R.layout.food_menu, planetList);



    // Set the ArrayAdapter as the ListView's adapter.
   list.setAdapter( listAdapter );

错误在底线 它说

  

java.lang.RuntimeException：无法启动活动   ComponentInfo {} com.example.grantjones20.restaurant/com.example.grantjones20.restaurant.MainActivity：   java.lang.NullPointerException：尝试调用虚方法＆＃39; void   android.widget.ListView.setAdapter（android.widget.ListAdapter）＆＃39;在...上   null对象引用