Nullability of reference types in type of parameter 'source' of 'void NotificationSystem.OnTimedEvent(object source, ElapsedEventArgs e)' doesn't match the target delegate 'ElapsedEventHandler' (possibly because of nullability attributes).

Non-nullable field 'data' must contain a non-null value when exiting constructor. Consider adding the 'required' modifier or declaring the field as nullable.

Converting null literal or possible null value to non-nullable type.

Possible null reference argument for parameter 'apiKey' in 'User AuthProvider.GetUser(string apiKey)'.

Dereference of a possibly null reference.

Converting null literal or possible null value to non-nullable type.

Non-nullable field '_timer' must contain a non-null value when exiting constructor. Consider adding the 'required' modifier or declaring the field as nullable.

Possible null reference return.

Possible null reference return.