Fix NPE when scheduling transaction for null client

Application process was crash and the application thread was set
to null when obtaining the client transaction object. NPE was
thrown by using it to schedule transaction.

Bug: 130006936
Test: WindowProcessControllerTests
Test: ActivityStartControllerTests
Test: RootActivityContainerTests

Change-Id: I65ecb124e014e5d80ebc283884475619f95239fa