SelectExtended: Option to Add Null Value into ItemCollection and Fix Disabled Visual - #568

Dereference of a possibly null reference.

Dereference of a possibly null reference.

Dereference of a possibly null reference.

Nullability of reference types in type of parameter 'sender' of 'void MudWatch.Elapse(object sender, ElapsedEventArgs args)' doesn't match the target delegate 'ElapsedEventHandler' (possibly because of nullability attributes).

Dereference of a possibly null reference.

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

Dereference of a possibly null reference.

Dereference of a possibly null reference.

The type 'T' cannot be used as type parameter 'TKey' in the generic type or method 'Dictionary<TKey, TValue>'. Nullability of type argument 'T' doesn't match 'notnull' constraint.