Avoid the scenario Where taskdisplayarea is null

Problem: Framework Reboot i.e., NPE in android.ui Thread
E AndroidRuntime: FATAL EXCEPTION IN SYSTEM PROCESS: android.ui
E AndroidRuntime: java.lang.NullPointerException: Attempt to invoke virtual method 'com.android.server.wm.ActivityStack com.android.server.wm.TaskDisplayArea.getRootHomeTask()' on a null object reference
E AndroidRuntime: **at com.android.server.wm.ActivityStarter.postStartActivityProcessing(ActivityStarter.java:1486)
E AndroidRuntime: **at com.android.server.wm.ActivityStarter.startActivityUnchecked(ActivityStarter.java:1533)
E AndroidRuntime: **at com.android.server.wm.ActivityStarter.executeRequest(ActivityStarter.java:1190)
E AndroidRuntime: **at com.android.server.wm.ActivityStarter.execute(ActivityStarter.java:673)
E AndroidRuntime: **at com.android.server.wm.ActivityStartController.startHomeActivity(ActivityStartController.java:206)

Analysis:
getDisplayArea in Task and its Super Class is annotated with Nullable and can be return Null.

Bug: 196568035
Test: StabilityTest
Change-Id: I4f3c8fdca0202591e7ac07aa7b8e3de3b08b2475