Avoid NullPointerExceltions by using a getter for wrapped values with @Nullable annotation

[HenrikJannsen]

We should use always the @Nullable annotation if a value can be null.
When using JavaFX properties or other swappers (Observeable) we cannot apply the @Nullable annotation as the wrapper is final and never null though the wrapped value can be null.
To avoid running into NullPointerExceltions we should use a getter method when accessing the value and annotate the getter method with @Nullable.

In the UI code we use direct field access as the model is static private and accessible. We should use getter methods instead to gain more control.
In case we have a nullable value we should postfix the property field with Property to make it more clearly distinguished from the getter.

        private final ObjectProperty<ChatChannel<? extends ChatMessage>> channelProperty = new SimpleObjectProperty<>();
        @Nullable
        public ChatChannel<? extends ChatMessage> getChannel() {
            return channelProperty.get();
        }

[DrColver]

Using getters, and setters, would be good. I added a setter to UserIdentityStore to ensure the correct usage of the selectedUserIdentity Observable. If the Property postfix scheme is to be used, then it would follow that Observable should be used as a postfix in corresponding situations.

[HenrikJannsen]

Yes probably best to follow the JavaFx property conventions.

[HenrikJannsen]

Not so sure anymore about that. When applying it to some new classes it did not felt like an improvement as the model classes get very boilerplate. Lombok helps to keep them minimal... Will need more time to get a clear opinion.

[DrColver]

It's true that you get a lot more boilerplate code which hampers readability. As long as the conventions are used consistently either way is good. Preferably over the entire project.

[HenrikJannsen]

I guess we can close that. The extra pain with boilerplate does not justify the benefits IMO.