Tree-/TableCell: NPE when accessing edit event from startEdit

failing test:

    @Test
    public void testEditStartEventAfterStartOnCell() {
        setupForEditing();
        int editingIndex = 1;
        cell.updateIndex(editingIndex);
        List<CellEditEvent<?, ?>> events = new ArrayList<>();
        editingColumn.setOnEditStart(events::add);
        cell.startEdit();
        assertEquals(editingColumn, events.get(0).getTableColumn());
    }

underlying reason is startEdit creating the event with an incorrect location:

            CellEditEvent<S,?> editEvent = new CellEditEvent<>(
                table,
                 table.getEditingCell(),
                TableColumn.editStartEvent(),
                null
            );

issue is that table might not yet be in editing state (JDK-8187474),
fix is similar to that of the corresponding issue for ListCell JDK-8187432 - use cell state to create the tablePosition

Note: the NPE (vs. having an incorrect location) is caused by JDK-8269871 which accesses properties of the position without null guard