Fixed the NullPointerException

The problem was caused by trying to reference a view by ID in the declaration of the member variable lv before ContentView has been set.
Moved the definition of lv into populateList().

Moreover, since this activity extends ListView, it needed to have an @android:id/list view element which was a @+id/list instead.
Fixed.

The @android:id/list cannnot be accessed with findViewById().
Used the getListView() function specified by the API.

Signed-off-by: Denis Torgunov <D.Torgunov@student.bradford.ac.uk>