Check null for top activity when updating orientation

Ideally if 'r' is non-null, topRunningActivity() should at least
find it. But there may be corner cases such as all activities in
the display are finishing, or other places didn't clear orientation
source correctly for an empty display.

Bug: 266896543
Test: DisplayContentTests
Change-Id: I86a1395e3fcfc3a0338cacd85256c442c5299b1d

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

@@ -1679,7 +1679,7 @@ class DisplayContent extends RootDisplayArea implements WindowManagerPolicy.Disp
             }
             // The orientation source may not be the top if it uses SCREEN_ORIENTATION_BEHIND.
             final ActivityRecord topCandidate = !r.isVisibleRequested() ? topRunningActivity() : r;
-            if (handleTopActivityLaunchingInDifferentOrientation(
+            if (topCandidate != null && handleTopActivityLaunchingInDifferentOrientation(
                     topCandidate, r, true /* checkOpening */)) {
                 // Display orientation should be deferred until the top fixed rotation is finished.
                 return false;