To reproduce, compile and run the example - a list with a null item at index 0:

- press remove null
- press add null
- increase window height a bit to show an additional row
- expected: all rows beyond items.size are empty
- actual: one of them rows shows "null"

The culprit is incomplete logic in updateItem(int):

if (valid) { // index in range
     ...
     updateItem(newItem, false)
} else {
    if (!isEmpty
        && oldItem != null) {// this lets cell with null content slip through
          updateItem(null, true);
    }

Fix seems to be to just remove the null check - off range cells must be marked empty always. Doing so leads to example behaving as expected, all tests still passing. Couldn't come up (yet) with a failing/passing unit test focused on this.

The same incorrect logic is used in all types of cells, might have similar issues (didn't try, though)

The example:

public class ListCellNullValue extends Application {
    private Parent createContent() {
        // instantiate list containing null item
        ListView<String> list = new ListView<>(FXCollections.observableArrayList((String)null, "item 1"));
        
        Button removeNull = new Button("remove null");
        removeNull.setOnAction(e -> {
            list.getItems().remove(0);
        });
        
        Button addNull = new Button("add null");
        addNull.setOnAction(e -> {
            list.getItems().add(0, null);
        });
        BorderPane content = new BorderPane(list);
        content.setBottom(new HBox(10, removeNull, addNull));
        return content;
    }

    @Override
    public void start(Stage stage) throws Exception {
        stage.setScene(new Scene(createContent()));
        stage.setTitle(FXUtils.version());
        stage.show();
    }

    public static void main(String[] args) {
        launch(args);
    }
}