[Divider] Fix a crash because of null renderer

Because the ending snap animation may take some time, the renderer can
become null during this process, causing NPE. This happens rarely and is
only observed when finishing an activity while dragging animation is
played.

Bug: 355252010
Test: atest DividerPresenterTest
Flag: com.android.window.flags.activity_embedding_interactive_divider_flag
Change-Id: Id32d7a7ce016d928bf409778bfe3f912bb89656f