Silently give up split action when mState is null

mState may be null if the Fragment is recreated (or in the other
strange state) by the system during split confirmation, which is
really rare for users (while it may happen during automated
testing). Avoid NPE and give up the save action silently.

Bug: 5220440
Change-Id: I40c9a29ef1f3c1a3913beb9f3a8022b00d297102