swift - iOS: Core Data light weight migration is throwing error -

in 1.0 version of app, have created core data db named "myapp.sqlite". when core data light weight migration same name "myapp.sqlite", getting below error

 terminating app due uncaught exception 'nsinternalinconsistencyexception' , reason: 'can't modify immutable model 

below code

// mark: - core data stack

lazy var applicationdocumentsdirectory: nsurl = {      let urls = nsfilemanager.defaultmanager().urlsfordirectory(.documentdirectory, indomains: .userdomainmask)     return urls[urls.count-1] }()  lazy var managedobjectmodel: nsmanagedobjectmodel = {     // managed object model application. property not optional. fatal error application not able find , load model.     let modelurl = nsbundle.mainbundle().urlforresource("myapp", withextension: "momd")!     return nsmanagedobjectmodel(contentsofurl: modelurl)! }()  lazy var persistentstorecoordinator: nspersistentstorecoordinator? = {     print("hello")     // persistent store coordinator application. implementation creates , return coordinator, having added store application it. property optional since there legitimate error conditions cause creation of store fail.     // create coordinator , store     var coordinator: nspersistentstorecoordinator? =  nspersistentstorecoordinator(managedobjectmodel: self.managedobjectmodel)     let url = self.applicationdocumentsdirectory.urlbyappendingpathcomponent("myapp.sqlite")     var err: nserror? = nil     var failurereason = "there error creating or loading application's saved data."     let options = [nsmigratepersistentstoresautomaticallyoption: true, nsinfermappingmodelautomaticallyoption: true]       nslog("url =---- \(url)")       {         try coordinator!.addpersistentstorewithtype(nssqlitestoretype, configuration: nil, url: url, options: options)     } catch let error nserror {         print(error.localizeddescription)          coordinator = nil         // report error got.         var dict = [string: anyobject]()         dict[nslocalizeddescriptionkey] = "failed initialize application's saved data"         dict[nslocalizedfailurereasonerrorkey] = failurereason         dict[nsunderlyingerrorkey] = error         err = nserror(domain: "your_error_domain", code: 9999, userinfo: dict)         // replace code handle error appropriately.         // abort() causes application generate crash log , terminate. should not use function in shipping application, although may useful during development.         nslog("unresolved error \(error), \(error.userinfo)")         abort()        }          return coordinator }()  lazy var managedobjectcontext: nsmanagedobjectcontext? = {     // returns managed object context application (which bound persistent store coordinator application.) property optional since there legitimate error conditions cause creation of context fail.     let coordinator = self.persistentstorecoordinator     if coordinator == nil {         return nil     }     var managedobjectcontext = nsmanagedobjectcontext()     managedobjectcontext.persistentstorecoordinator = coordinator     return managedobjectcontext }()