Fix exception when trying to update config of detached stack

There is an NullPointerException which can be reproduced with following steps
on angler:
1. In split-screen open activities from the same app.
   Make sure to launch them from launcher.
2. Rotate device to landscape.
3. Force-stop the app using adb command.

After killing the app it will try to open launcher, which is fixed in
portrait mode on that device. This will update the configuration for all the
stacks, including already detached.

Bug: 28786255
Change-Id: Ia326c09cff9f1a3fb43cca5b48db29718553346a