Error ListView in findViewById

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http://stackoverflow.com – The class is an extension of another class and is experiencing an error when I mess with the listview. Can anyone help me? Layout: GridLayout XML: <ListView android:id="@+id/lvContexts" android:layout_width="899dp" android:layout_height="356dp" android:layout_column="1" android:layout_columnSpan="3" android:layout_gravity="left" android:layout_row="3" android:layout_rowSpan="2" > </ListView> Class: public class EditarContext extends Iniciar { .... lvContext = (ListView) findViewById(R.id.lvContexts); ArrayList<String> lvList = (Ar (HowTos)