Your XML file is wrong, you have spelled textview in all lower case (
textview
instead of
TextView
), and your opening tag has a closing brace (>). It should be:
<TextView
android:id="@+id/main_textview"
android:text="@string/textview"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginLeft="20dp"
android:layout_marginTop="20dp"
/>