Prevent system server crash while start activity failed.

NPE at PendingActivityLaunch#sendErrorResult, callerApp can be null.

Bug: 135648362
Test: atest ActivityStarterTests
Change-Id: I7949dc1effcc171834f6bc3ff0333f9090b58480

services/core/java/com/android/server/wm/ActivityStackSupervisor.java

@@ -415,7 +415,7 @@ public class ActivityStackSupervisor implements RecentTasks.Callbacks {
 
         void sendErrorResult(String message) {
             try {
-                if (callerApp.hasThread()) {
+                if (callerApp != null && callerApp.hasThread()) {
                     callerApp.getThread().scheduleCrash(message);
                 }
             } catch (RemoteException e) {