RFR: 8279640: ListView with null SelectionModel/FocusModel throws NPE

[Kevin Rushforth]

On Sat, 8 Jan 2022 00:17:36 GMT, Marius Hanl <mhanl at openjdk.org> wrote:

> This PR fixes a bunch of NPEs when a null `SelectionModel` or `FocusModel` is set on a `ListView`.
> 
> The following NPEs are fixed (all are also covered by exactly one test case):
> NPEs with null selection model:
> - Mouse click on a `ListCell`
> - SPACE key press
> - KP_UP (arrow up) key press
> - HOME key press
> - END key press
> - BACK_SLASH + CTRL key press
> 
> NPEs with null focus model:
> - SPACE key press
> - Select an items: getSelectionModel().select(1)
> - Clear-Select an item and add one after: listView.getSelectionModel().clearAndSelect(1); listView.getItems().add("3");

I'd like to see this get resolved for JavaFX 20. I note there is a similar issue with `TableView`, which is tracked by [JDK-8187145](https://bugs.openjdk.org/browse/JDK-8187145).

As I mentioned in that JBS issue, I tend to agree that if we were starting today with a blank sheet of paper, we might have disallowed null and defined a "noop" selection model and/or focus model. At this point, though, it would be a breaking change, even though we don't specify the behavior of null, so we ought to make it work in those places where it doesn't.

> So there is no consistent behaviour for this, but a lot of code already handles null by behaving in some kind of default way without changing the property directly, and I think it might be the best to adapt to this.

This seems like the best way forward to me, but let's see what comes out of the review.

As part of this, we should clarify the spec that a null selection (and focus) model is allowed, and say what it does. This might mean a CSR, but I'd want to look at the current docs first.

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PR: https://git.openjdk.org/jfx/pull/711