open-event-organizer-android: App crashing on Events Refresh

• Events refresh causes a Foreign Key constraint failure
• The migration should be set up correctly so that user does not have to clear the app on upgrades

@mejariamol Please take a look. This is what I was talking about while deleting the events

Log:

FATAL EXCEPTION: RxCachedThreadScheduler-2
  Process: org.fossasia.openevent.app, PID: 3435
  io.reactivex.exceptions.OnErrorNotImplementedException: FOREIGN KEY constraint failed (code 787)
      at io.reactivex.internal.observers.EmptyCompletableObserver.onError(EmptyCompletableObserver.java:50)
      at io.reactivex.internal.operators.completable.CompletableConcatArray$ConcatInnerObserver.onError(CompletableConcatArray.java:60)
      at io.reactivex.internal.operators.completable.CompletableFromAction.subscribeActual(CompletableFromAction.java:38)
      at io.reactivex.Completable.subscribe(Completable.java:1635)
      at io.reactivex.internal.operators.completable.CompletableConcatArray$ConcatInnerObserver.next(CompletableConcatArray.java:89)
      at io.reactivex.internal.operators.completable.CompletableConcatArray.subscribeActual(CompletableConcatArray.java:33)
      at io.reactivex.Completable.subscribe(Completable.java:1635)
      at io.reactivex.Completable.subscribe(Completable.java:1623)
      at org.fossasia.openevent.app.data.EventRepository.lambda$null$11(EventRepository.java:117)
      at org.fossasia.openevent.app.data.EventRepository$$Lambda$19.accept(Unknown Source)
      at io.reactivex.internal.operators.observable.ObservableDoOnEach$DoOnEachObserver.onNext(ObservableDoOnEach.java:95)
      at retrofit2.adapter.rxjava2.BodyObservable$BodyObserver.onNext(BodyObservable.java:51)
      at retrofit2.adapter.rxjava2.BodyObservable$BodyObserver.onNext(BodyObservable.java:37)
      at retrofit2.adapter.rxjava2.CallExecuteObservable.subscribeActual(CallExecuteObservable.java:43)
      at io.reactivex.Observable.subscribe(Observable.java:10842)
      at retrofit2.adapter.rxjava2.BodyObservable.subscribeActual(BodyObservable.java:34)
      at io.reactivex.Observable.subscribe(Observable.java:10842)
      at io.reactivex.internal.operators.observable.ObservableDoOnEach.subscribeActual(ObservableDoOnEach.java:42)
      at io.reactivex.Observable.subscribe(Observable.java:10842)
      at io.reactivex.internal.operators.observable.ObservableFlattenIterable.subscribeActual(ObservableFlattenIterable.java:44)
      at io.reactivex.Observable.subscribe(Observable.java:10842)
      at io.reactivex.internal.operators.observable.ObservableDefer.subscribeActual(ObservableDefer.java:39)
      at io.reactivex.Observable.subscribe(Observable.java:10842)
      at io.reactivex.internal.operators.observable.ObservableDoOnEach.subscribeActual(ObservableDoOnEach.java:42)
      at io.reactivex.Observable.subscribe(Observable.java:10842)
      at io.reactivex.internal.operators.observable.ObservableDoOnEach.subscribeActual(ObservableDoOnEach.java:42)
      at io.reactivex.Observable.subscribe(Observable.java:10842)
      at io.reactivex.internal.operators.observable.ObservableScalarXMap$ScalarXMapObservable.subscribeActual(ObservableScalarXMap.java:166)
      at io.reactivex.Observable.subscribe(Observable.java:10842)
      at io.reactivex.internal.operators.observable.ObservableSwitchIfEmpty$SwitchIfEmptyObserver.onComplete(ObservableSwitchIfEmpty.java:70)
      at io.reactivex.internal.disposables.EmptyDisposable.complete(EmptyDisposable.java:53)
      at io.reactivex.internal.operators.observable.ObservableEmpty.subscribeActual(ObservableEmpty.java:28)
      at io.reactivex.Observable.subscribe(Observable.java:10842)
      at io.reactivex.internal.operators.observable.ObservableDefer.subscribeActual(ObservableDefer.java:39)
      at io.reactivex.Observable.subscribe(Observable.java:10842)
      at io.reactivex.internal.operators.observable.ObservableSwitchIfEmpty.subscribeActual(ObservableSwitchIfEmpty.java:31)
      at io.reactivex.Observable.subscribe(Observable.java:10842)
      at io.reactivex.internal.operators.observable.ObservableSubscribeOn$SubscribeTask.run(ObservableSubscribeOn.java:96)
      at io.reactivex.Scheduler$DisposeTask.run(Scheduler.java:452)
      at io.reactivex.internal.schedulers.ScheduledRunnable.run(ScheduledRunnable.java:61)
      at io.reactivex.internal.schedulers.ScheduledRunnable.call(ScheduledRunnable.java:52)
      at java.util.concurrent.FutureTask.run(FutureTask.java:237)
07-06 15:23:13.943 3435-3507/org.fossasia.openevent.app E/AndroidRuntime:     at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:272)
      at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1133)
      at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:607)
      at java.lang.Thread.run(Thread.java:761)
   Caused by: android.database.sqlite.SQLiteConstraintException: FOREIGN KEY constraint failed (code 787)
      at android.database.sqlite.SQLiteConnection.nativeExecuteForChangedRowCount(Native Method)
      at android.database.sqlite.SQLiteConnection.executeForChangedRowCount(SQLiteConnection.java:734)
      at android.database.sqlite.SQLiteSession.executeForChangedRowCount(SQLiteSession.java:754)
      at android.database.sqlite.SQLiteStatement.executeUpdateDelete(SQLiteStatement.java:64)
      at com.raizlabs.android.dbflow.structure.database.AndroidDatabaseStatement.executeUpdateDelete(AndroidDatabaseStatement.java:39)
      at com.raizlabs.android.dbflow.sql.language.BaseModelQueriable.executeUpdateDelete(BaseModelQueriable.java:111)
      at com.raizlabs.android.dbflow.sql.language.BaseModelQueriable.executeUpdateDelete(BaseModelQueriable.java:106)
      at com.raizlabs.android.dbflow.sql.language.Delete.table(Delete.java:21)
      at org.fossasia.openevent.app.data.db.DatabaseRepository.lambda$deleteAll$9(DatabaseRepository.java:83)
      at org.fossasia.openevent.app.data.db.DatabaseRepository$$Lambda$10.run(Unknown Source)
      at io.reactivex.internal.operators.completable.CompletableFromAction.subscribeActual(CompletableFromAction.java:34)
      	... 43 more