You created and array, but there is nothing in it. Array field "table[0]" contains null.
Add there object like this:
tables[0] = new PdfPTable();
Or use a different contructor than PdfPTable(), according to what you exactly need
Here is your code updated
com.lowagie.text.pdf.PdfPTable[] table = new com.lowagie.text.pdf.PdfPTable[1];
tables[0] = new PdfPTable();
// the cell object
com.lowagie.text.pdf.PdfPCell cell;
// header
cell = new PdfPCell(new Phrase(wdComponentAPI.getMessage("Ordernr")));
cell.setColspan(1);
cell.setBackgroundColor(Color.LIGHT_GRAY);
cell.setHorizontalAlignment(Element.ALIGN_CENTER);
table[0].addCell(cell);