title: Cannot save a ParseUser that is not authenticated
body: <p>I'm trying to grab a ParseRelation of user's who have joined a group. I had no problem until I added login with Facebook. Now, I get the error of "Cannot save a ParseUser that is not authenticated" when trying to add a user to the relation and then saving the ParseObject. I can join groups created by people who did not log in with Facebook and I can join groups that I created (I'm logged in with FB), but get the error with group's created by another user who logged in with Facebook.</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false">
  <div class="snippet-code">
  <pre class="snippet-code-js lang-js prettyprint-override"><code>mJoinButton.setOnClickListener(new View.OnClickListener() {@
    Override
    public void onClick(View v) {
      ParseObject group = mGroups.get(getAdapterPosition());
      mJoinedRelation = group.getRelation(ParseConstants.KEY_JOINED_RELATION);
      mJoinedRelation.add(mCurrentUser);
      group.saveInBackground(new SaveCallback() {
  
        @
        Override
        public void done(ParseException e) {
          if (e == null) {
            Toast.makeText(mContext, "You have joined the group!", Toast.LENGTH_LONG).show();
  
          } else {
            AlertDialog.Builder builder = new AlertDialog.Builder(mContext)
              .setMessage(e.getMessage())
              .setTitle(R.string.login_error_title)
              .setPositiveButton(android.R.string.ok, null);
            AlertDialog dialog = builder.create();
            dialog.show();
          }
        }
      });
    }
  });</code></pre>
  </div>
  </div>
  </p>