fix NPE

fixes this NPE:

```
E/AndroidRuntime( 8322): java.lang.NullPointerException
E/AndroidRuntime( 8322): 	at android.widget.ArrayAdapter.init(ArrayAdapter.java:314)
E/AndroidRuntime( 8322): 	at android.widget.ArrayAdapter.<init>(ArrayAdapter.java:153)
E/AndroidRuntime( 8322): 	at nl.pilight.illumina.widget.DeviceAdapter.<init>(DeviceAdapter.java:104)
E/AndroidRuntime( 8322): 	at nl.pilight.illumina.fragment.DeviceListFragment.onLocationResponse(DeviceListFragment.java:116)
E/AndroidRuntime( 8322): 	at nl.pilight.illumina.service.PilightBinder$IncomingHandler.handleMessage(PilightBinder.java:112)
E/AndroidRuntime( 8322): 	at android.os.Handler.dispatchMessage(Handler.java:99)
E/AndroidRuntime( 8322): 	at android.os.Looper.loop(Looper.java:132)
E/AndroidRuntime( 8322): 	at android.app.ActivityThread.main(ActivityThread.java:4025)
E/AndroidRuntime( 8322): 	at java.lang.reflect.Method.invokeNative(Native Method)
E/AndroidRuntime( 8322): 	at java.lang.reflect.Method.invoke(Method.java:491)
E/AndroidRuntime( 8322): 	at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:841)
E/AndroidRuntime( 8322): 	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:599)
E/AndroidRuntime( 8322): 	at dalvik.system.NativeStart.main(Native Method)
```

getActivity() might be null at this point - this fixes the crash - but it is more like a patch that might cover a underlying structural problem I have not time to investigate at the moment