E2-1673 prevent null exception in LeanplumInbox (#281)

* add null check

* make vars volatile

Author: gg4race

AndroidSDKCore/src/main/java/com/leanplum/LeanplumInbox.java

@@ -61,9 +61,9 @@ public class LeanplumInbox {
   static Set<String> downloadedImageUrls;
   static boolean isInboxImagePrefetchingEnabled = true;
 
-  private int unreadCount;
-  private Map<String, LeanplumInboxMessage> messages;
-  private boolean didLoad = false;
+  private volatile int unreadCount;
+  private volatile Map<String, LeanplumInboxMessage> messages;
+  private volatile boolean didLoad = false;
 
   private final List<InboxChangedCallback> changedCallbacks;
   private final List<InboxSyncedCallback> syncedCallbacks;
@@ -430,7 +430,10 @@ private List<LeanplumInboxMessage> allMessages(List<LeanplumInboxMessage> messag
     }
     try {
       for (String messageId : messagesIds()) {
-        messages.add(messageForId(messageId));
+        LeanplumInboxMessage message = messageForId(messageId);
+        if (message != null) {
+          messages.add(message);
+        }
       }
     } catch (Throwable t) {
       Util.handleException(t);