Fix NPE on startup.

There is no invariant in the controller that the fragments it references
are non-null, so querying the fragment for state is not always valid.

Change-Id: I880053be4000260b03b54ed8741e646b01314e0d

src/com/android/email/activity/UIControllerTwoPane.java

@@ -487,7 +487,9 @@ class UIControllerTwoPane implements
      * for the mailbox list. The two may be different.
      */
     public long getMessageListMailboxId() {
-        return mMessageListFragment.getMailboxId();
+        return (mMessageListFragment == null)
+                ? Mailbox.NO_MAILBOX
+                : mMessageListFragment.getMailboxId();
     }
 
     public long getMessageId() {