Fix IWallpaperEngineWrapper NPE in wearOS

A crash was detected in wearOS. Simply add a null check to avoid that.

Bug: 282156194
Test: treehugger (very safe, just a null check)
Change-Id: I016c3c942b1b0099eaa423cad4b4d60c90804c79
diff --git a/core/java/android/service/wallpaper/WallpaperService.java b/core/java/android/service/wallpaper/WallpaperService.java
index 230f511..7f0a666 100644
--- a/core/java/android/service/wallpaper/WallpaperService.java
+++ b/core/java/android/service/wallpaper/WallpaperService.java
@@ -2231,7 +2231,7 @@
 
             mDestroyed = true;
 
-            if (mIWallpaperEngine.mDisplayManager != null) {
+            if (mIWallpaperEngine != null && mIWallpaperEngine.mDisplayManager != null) {
                 mIWallpaperEngine.mDisplayManager.unregisterDisplayListener(mDisplayListener);
             }