Add null checks to prevent NPE on ViewModel.

ViewModel can be null if onRecycle completes before coroutineJob is cancelled.

Fix: 395751707
Flag: Exempt - test fix
Test: OpenActivityEmbeddingPlaceholderSplitTest in postsubmit
Change-Id: I12e1f6e2e5951ae04264fc8ea9d45b6ac6d43308