fix(inbox): fixing null pointer exception

Author: milos1290

AndroidSDK/src/com/leanplum/LeanplumInbox.java

@@ -306,9 +306,18 @@ public List<String> messagesIds() {
     try {
       Collections.sort(messageIds, new Comparator<String>() {
         @Override
-        public int compare(String firstMessage, String secondMessage) {
-          Date firstDate = messageForId(firstMessage).getDeliveryTimestamp();
-          Date secondDate = messageForId(secondMessage).getDeliveryTimestamp();
+        public int compare(String firstMessageId, String secondMessageId) {
+          LeanplumInboxMessage firstMessage = messageForId(firstMessageId);
+          LeanplumInboxMessage secondMessage = messageForId(secondMessageId);
+          // Message that is null will be moved to the back of the list.
+          if (firstMessage == null) {
+            return -1;
+          }
+          if (secondMessage == null) {
+            return 1;
+          }
+          Date firstDate = firstMessage.getDeliveryTimestamp();
+          Date secondDate = secondMessage.getDeliveryTimestamp();
           return firstDate.compareTo(secondDate);
         }
       });

AndroidSDK/src/com/leanplum/NewsfeedMessage.java

@@ -149,6 +149,9 @@ public Date deliveryTimestamp() {
    * Returns the delivery timestamp of the newsfeed message.
    */
   public Date getDeliveryTimestamp() {
+    if (deliveryTimestamp == null) {
+      return new Date();
+    }
     return new Date(deliveryTimestamp);
   }