Add monkey protection to editUrl

    Bug: 5145898
    check for valid active tab

Change-Id: I32da53b71771b0f81af44fbbc317d6c891741098
diff --git a/src/com/android/browser/BaseUi.java b/src/com/android/browser/BaseUi.java
index 368021a..c1f8bfb 100644
--- a/src/com/android/browser/BaseUi.java
+++ b/src/com/android/browser/BaseUi.java
@@ -426,7 +426,7 @@
             mUiController.endActionMode();
         }
         showTitleBar();
-        if (!getActiveTab().isSnapshot()) {
+        if ((getActiveTab() != null) && !getActiveTab().isSnapshot()) {
             mNavigationBar.startEditingUrl(clearInput);
         }
     }