fix NPE when closing tab

    Bug: 3041603

Change-Id: Ifc77c576f072e5c78bf716f00d0efcb5b3ec2e86
diff --git a/src/com/android/browser/TabBar.java b/src/com/android/browser/TabBar.java
index 432df9f..b2c687d 100644
--- a/src/com/android/browser/TabBar.java
+++ b/src/com/android/browser/TabBar.java
@@ -419,9 +419,11 @@
     @Override
     public void onRemoveTab(Tab tab) {
         TabViewData tvd = mTabMap.get(tab);
-        TabView tv = tvd.mTabView;
-        if (tv != null) {
-            mTabs.removeTab(tv);
+        if (tvd != null) {
+            TabView tv = tvd.mTabView;
+            if (tv != null) {
+                mTabs.removeTab(tv);
+            }
         }
         mTabMap.remove(tab);
     }