frameworks/base: Handle null from topRunningNonDelayedActivityLocked

startActivityUncheckedLocked tries to move the target task to front
when it is not at front. topRunningNonDelayedActivityLocked is used
to find the current task, however null value isn't handled. This
null causes an unhandled exception leading to the android framework
reboot.

Change-Id: I2a43cda50483e28a4456846d8b3ccb30d7cf110e

Author: Ethan1983

services/java/com/android/server/am/ActivityManagerService.java

@@ -3410,7 +3410,7 @@ private final int startActivityUncheckedLocked(HistoryRecord r,
                     // being started, which means not bringing it to the front
                     // if the caller is not itself in the front.
                     HistoryRecord curTop = topRunningNonDelayedActivityLocked(notTop);
-                    if (curTop.task != taskTop.task) {
+                    if (curTop != null && curTop.task != taskTop.task) {
                         r.intent.addFlags(Intent.FLAG_ACTIVITY_BROUGHT_TO_FRONT);
                         boolean callerAtFront = sourceRecord == null
                                 || curTop.task == sourceRecord.task;