Add valid check for webview before starting to download

Change-Id: I9c68f7fa95823f847a8616bc5c2e247855e0393f
diff --git a/src/com/android/browser/Controller.java b/src/com/android/browser/Controller.java
index 5d8ac35..4d72320 100644
--- a/src/com/android/browser/Controller.java
+++ b/src/com/android/browser/Controller.java
@@ -1158,6 +1158,7 @@
             String contentDisposition, String mimetype, String referer,
             long contentLength) {
         WebView w = tab.getWebView();
+        if ( w == null) return;
         boolean ret = DownloadHandler.onDownloadStart(mActivity, url, userAgent,
                 contentDisposition, mimetype, referer, w.isPrivateBrowsingEnabled(), contentLength);
         if (ret == false && w.copyBackForwardList().getSize() == 0) {