Avoid NPE from delayed Runnable

Naive fix to b/12656485. Other Runnables in Launcher similarly check
that mWorkspace hasn't become null between being posted and being
executed, so I think this is probably appropriate.

Bug: 12656485
Change-Id: Ib4f3d0e8686c6249fe838144b1e8120c85f66f01
1 file changed