Added error handling for nil callbacks
Added error handling for nil callbacks

Duckr/SwiftDDP,siegesmund/SwiftDDP,veackell/SwiftDDP,infinitedg/SwiftDDP,mjgaylord/SwiftDDP,veackell/SwiftDDP,siegesmund/RealmMeteor,siegesmund/RealmMeteor,Duckr/SwiftDDP,mjgaylord/SwiftDDP,veackell/SwiftDDP,infinitedg/SwiftDDP,siegesmund/SwiftDDP