Fix null ref

Fixes bug 2261398.

Change-Id: Idb4db938e93b792248d49410b9474c71d5d4bb37
diff --git a/src/com/android/mms/ui/ComposeMessageActivity.java b/src/com/android/mms/ui/ComposeMessageActivity.java
index d5c0495..2eb4dd7 100644
--- a/src/com/android/mms/ui/ComposeMessageActivity.java
+++ b/src/com/android/mms/ui/ComposeMessageActivity.java
@@ -1781,7 +1781,7 @@
             log("     new conversation=" + conversation + ", mConversation=" + mConversation);
         }
 
-        long convThreadId = conversation.getThreadId();
+        long convThreadId = conversation == null ? 0 : conversation.getThreadId();
         if (sameThread || (convThreadId != 0 && convThreadId == mConversation.getThreadId())) {
             if (Log.isLoggable(LogTag.APP, Log.VERBOSE)) {
                 log("onNewIntent: same conversation");