fix NPE in instant suggestions

    Bug: 3510741
    check for null searchbox which might be returned
    by change I256375b2

Change-Id: Ic40209f2c1ea9a87098ce11586b4fd92f9065559
diff --git a/src/com/android/browser/InstantSearchEngine.java b/src/com/android/browser/InstantSearchEngine.java
index 1d9bdd6..128997c 100644
--- a/src/com/android/browser/InstantSearchEngine.java
+++ b/src/com/android/browser/InstantSearchEngine.java
@@ -185,7 +185,9 @@
                 mListener.clear();
             }
             mSearchBox = searchBox;
-            mSearchBox.addSearchBoxListener(mListener);
+            if (mSearchBox != null) {
+                mSearchBox.addSearchBoxListener(mListener);
+            }
         }
     }