NPE in FeedFragment

The app crashed after using the back button to leave the blog feed. The stacktrace comes from a branch, so the line numbers may be slightly different from those on master, but as far as I can see the exception was caused by getActivity() returning null, which would happen in the same way on master.

08-02 18:18:01.398 12121-12165/org.briarproject E/ACRA: ACRA caught a NullPointerException for org.briarproject
java.lang.NullPointerException
  at org.briarproject.android.blogs.FeedFragment.onBlogPostAdded(FeedFragment.java:164)
  at org.briarproject.android.blogs.FeedControllerImpl$3.run(FeedControllerImpl.java:144)
  at org.briarproject.android.controller.DbControllerImpl$1.run(DbControllerImpl.java:35)
  at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1088)
  at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:581)
  at java.lang.Thread.run(Thread.java:1027)