Add null pointer judgment processing to prevent restarting

When getDisplayArea() is not default display,there will be a null pointer situation,
to avoid this situation, a null pointer judgment has been added.

Bug:295103280
Test:Stability test

Change-Id: I46c1463bab7889ba0395c67b8bd1ffa1148326cf
Signed-off-by: HanWang <hanwang627@gmail.com>
diff --git a/services/core/java/com/android/server/wm/Task.java b/services/core/java/com/android/server/wm/Task.java
index 91c0c69..bda0a0f 100644
--- a/services/core/java/com/android/server/wm/Task.java
+++ b/services/core/java/com/android/server/wm/Task.java
@@ -5748,7 +5748,7 @@
                 false /* deferResume */);
 
         ActivityRecord topActivity = getDisplayArea().topRunningActivity();
-        Task topRootTask = topActivity.getRootTask();
+        Task topRootTask = topActivity == null ? null : topActivity.getRootTask();
         if (topRootTask != null && topRootTask != this && topActivity.isState(RESUMED)) {
             // Usually resuming a top activity triggers the next app transition, but nothing's got
             // resumed in this case, so we need to execute it explicitly.