Fix null pointer exception when updating action bar shadow.

Check for valid activity when trying to access the action bar as the
activity can become null when in monkey test.

Change-Id: I684d873b9eabb9d8461e99bb4385d411a48c0c52
Fix: 64084651
Test: make RunSettingsRoboTests

src/com/android/settings/widget/ActionBarShadowController.java

@@ -121,7 +121,7 @@ public class ActionBarShadowController implements LifecycleObserver, OnStart, On
             final boolean shouldShowShadow = view.canScrollVertically(-1);
             if (mAnchorView != null) {
                 mAnchorView.setElevation(shouldShowShadow ? ELEVATION_HIGH : ELEVATION_LOW);
-            } else {
+            } else if (mActivity != null) { // activity can become null when running monkey
                 final ActionBar actionBar = mActivity.getActionBar();
                 if (actionBar != null) {
                     actionBar.setElevation(shouldShowShadow ? ELEVATION_HIGH : ELEVATION_LOW);

tests/robotests/src/com/android/settings/widget/ActionBarShadowControllerTest.java

@@ -104,4 +104,15 @@ public class ActionBarShadowControllerTest {
         verify(mRecyclerView, times(2)).addOnScrollListener(any());
     }
 
+    @Test
+    public void onScrolled_nullAnchorViewAndActivity_shouldNotCrash() {
+        final Activity activity = null;
+        final ActionBarShadowController controller =
+                ActionBarShadowController.attachToRecyclerView(activity, mLifecycle, mRecyclerView);
+
+        // Scroll
+        controller.mScrollChangeWatcher.onScrolled(mRecyclerView, 10 /* dx */, 10 /* dy */);
+        // no crash
+    }
+
 }