From the SimpleDB documentation
The sort attribute must be present in at least one of the predicates of the expression.
This means you will probably need to change your query to:
select * FROM ContentMaster where ContentAddedByUserID='%@' AND HiveletCode='%@' AND ContentAddedDateTime > 0 ORDER BY ContentAddedDateTime DESC
As for the crash, this an exception from the SDK, you should either disable exceptions and handle errors or put your calls in a @try/@catch block. You can read more about how exceptions and errors are handled in the AWS SDK for iOS in our blog.