Since fx9, access to tableSkin changed from protected to package-private. Also its accessibility in the headers' life-cycle changed from initially in super constructor (that is very early) to after setting its TableHeaderRow (<aside> wondering why you didn't keep it as a field in TableColumnHeader?</aside>)

The tableSkin is needed in (not only) custom headers to configure themselves from skin/table properties. Core headers now configure themselves in

    void setTableHeaderRow(TableHeaderRow thr) {
        tableHeaderRow = thr;
        updateTableSkin();
    }

    private void updateTableSkin() {
        // when we get the table header row, we are also given the skin,
        // so this is the time to hook up listeners, etc.
        TableViewSkinBase<?,?,?,?,?> tableSkin = getTableSkin();
        if (tableSkin == null) return;
        // do config stuff related to skin/table
       ...
    }

    TableViewSkinBase<?,?,?,?,?> getTableSkin() {
        return tableHeaderRow == null ? null : tableHeaderRow.tableSkin;
    }

For custom headers, there is no way to reliably hook into the config: even with deep reflection for accessing the package/private methods, the time as to when the skin is available is not clearly defined (for subs).

The easiest solution (for me as application/framework developer) would be to move those methods into protected scope ...