Fix crash in FileChooserFillTask.onPreExecute()

A similar fix was done to VolumeBrowserActivity$FillTask.onPreExecute()
but wasn't applied to FileChooserFillTask.onPreExecute(). In case the
user spams refresh the ListView can return null from getParent(), which
leads to a NullPointerException. The fix is to not create a progress bar
in these cases.