Fix NPE when trying to access task before AppWindowToken attach to one

This is a race condition. While create AppWindowToken, before attach to
a task container.addChild(), accessing task attached to the token will
cause an NPE. The root cause can be fixed after we unifying Task and
TaskRecord. Just post a temporary solution here to avoid crash.

Bug: 120197396
Test: go/wm-smoke
Change-Id: I37008c16546e02358d9f88d1c9fd544002357137