fix monkey NPE

    Bug: 5372741

Change-Id: I9e7461371f1f801e33d890329f820b7440c6d865
diff --git a/src/com/android/browser/NavigationBarTablet.java b/src/com/android/browser/NavigationBarTablet.java
index 04f372a..026e903 100644
--- a/src/com/android/browser/NavigationBarTablet.java
+++ b/src/com/android/browser/NavigationBarTablet.java
@@ -261,7 +261,9 @@
         if (mTitleBar.isInLoad()) {
             mUiController.stopLoading();
         } else {
-            mUiController.getCurrentTopWebView().reload();
+            if (mUiController.getCurrentTopWebView() != null) {
+                mUiController.getCurrentTopWebView().reload();
+            }
         }
     }