Fix a monkey crash.

Since getTopWindow() can be null, check first before accessing it.
Fix for http://b/issue?id=2027301
diff --git a/src/com/android/browser/BrowserActivity.java b/src/com/android/browser/BrowserActivity.java
index c4c8d33..66832ed 100644
--- a/src/com/android/browser/BrowserActivity.java
+++ b/src/com/android/browser/BrowserActivity.java
@@ -1555,7 +1555,7 @@
      */
     @Override
     public boolean onSearchRequested() {
-        String url = getTopWindow().getUrl();
+        String url = (getTopWindow() == null) ? null : getTopWindow().getUrl();
         startSearch(mSettings.getHomePage().equals(url) ? null : url, true,
                 createGoogleSearchSourceBundle(GOOGLE_SEARCH_SOURCE_SEARCHKEY), false);
         return true;