bug:
1. click home to hide app.
2. after several hours, click app icon to get it back.
3. path show null, listview is empty.

I guess:
1. when app hide, fragment is destroy.
2. when app back to frant, fragment is recreated. but at this time, not
sure adapter is set before or after. if adapter is set after, listview
can't get data to show.

resolution:
when fragment find adapter is null, call back to activity to set
adapter. 

having use two phone to test once, it seems ok, but i think it need
further test.