Prevent crash

app/src/main/java/eu/faircode/email/Core.java

@@ -1916,7 +1916,8 @@ class Core {
         } else {
             if (imessages != null && imessages.length > 1)
                 Log.e(folder.name + " EXISTS messages=" + imessages.length + " retry=" + retry);
-            EntityLog.log(context, folder.name + " EXISTS messages=" + imessages.length);
+            EntityLog.log(context, folder.name +
+                    " EXISTS messages=" + (imessages == null ? null : imessages.length));
             EntityOperation.queue(context, message, EntityOperation.ADD);
         }
     }