Adding null check before delegating touch event. (Bug 6296855)

Change-Id: I64ae955b2748969568ebbb4c39e00ce4b3619c6f
diff --git a/src/com/android/noisefield/NoiseFieldWallpaper.java b/src/com/android/noisefield/NoiseFieldWallpaper.java
index 08a6769..3b457ef 100644
--- a/src/com/android/noisefield/NoiseFieldWallpaper.java
+++ b/src/com/android/noisefield/NoiseFieldWallpaper.java
@@ -95,7 +95,9 @@
         @Override
         public void onTouchEvent(MotionEvent ev) {
             super.onTouchEvent(ev);
-            mWallpaperRS.onTouchEvent(ev);
+            if (mWallpaperRS != null) {
+                mWallpaperRS.onTouchEvent(ev);
+            }
         }
 
         @Override