Fix Monkey NPE

    Bug: 5177427

Change-Id: I7cfcfe63d8ba64b30a3fbe6a0c46efd15dc8688b
diff --git a/src/com/android/browser/PhoneUi.java b/src/com/android/browser/PhoneUi.java
index 49419aa..7d92cf4 100644
--- a/src/com/android/browser/PhoneUi.java
+++ b/src/com/android/browser/PhoneUi.java
@@ -319,9 +319,11 @@
     }
 
     private void finishAnimationIn() {
-        // notify accessibility manager about the screen change
-        mNavScreen.sendAccessibilityEvent(AccessibilityEvent.TYPE_WINDOW_STATE_CHANGED);
-        mTabControl.setOnThumbnailUpdatedListener(mNavScreen);
+        if (mNavScreen != null) {
+            // notify accessibility manager about the screen change
+            mNavScreen.sendAccessibilityEvent(AccessibilityEvent.TYPE_WINDOW_STATE_CHANGED);
+            mTabControl.setOnThumbnailUpdatedListener(mNavScreen);
+        }
     }
 
     void hideNavScreen(boolean animate) {