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

core/java/android/service/wallpaper/WallpaperService.java

@@ -2231,7 +2231,7 @@ public abstract class WallpaperService extends Service {
 
             mDestroyed = true;
 
-            if (mIWallpaperEngine.mDisplayManager != null) {
+            if (mIWallpaperEngine != null && mIWallpaperEngine.mDisplayManager != null) {
                 mIWallpaperEngine.mDisplayManager.unregisterDisplayListener(mDisplayListener);
             }