When creating invoice from a purchase order, with any Invoicing Control Method, it will show an Uncaught exception if the product field is left empty. This is due to the fact that the Invoice creation needs the "Expense Account" for each item, information that OE takes from the product itself or its parent categories:

2015-09-24 15:14:23,720 4952 ERROR TOSA_DEBUG openerp.osv.osv: Uncaught exception

Traceback (most recent call last):

  File "C:\Users\Nanni\git\odoo\openerp\osv\osv.py", line 132, in wrapper

    return f(self, dbname, *args, **kwargs)

  File "C:\Users\Nanni\git\odoo\openerp\osv\osv.py", line 199, in execute

    res = self.execute_cr(cr, uid, obj, method, *args, **kw)

  File "C:\Users\Nanni\git\odoo\openerp\osv\osv.py", line 187, in execute_cr

    return getattr(object, method)(cr, uid, *args, **kw)

  File "C:\Users\Nanni\git\odoo\addons\purchase\wizard\purchase_line_invoice.py", line 99, in makeInvoices

    acc_id = purchase_obj._choose_account_from_po_line(cr, uid, line, context=context)

  File "C:\Users\Nanni\git\odoo\addons\purchase\purchase.py", line 502, in _choose_account_from_po_line

    acc_id = property_obj.get(cr, uid, 'property_account_expense_categ', 'product.category', context=context).id

AttributeError: 'bool' object has no attribute 'id'

2015-09-24 15:14:23,720 4952 ERROR TOSA_DEBUG openerp.netsvc: 'bool' object has no attribute 'id'

Traceback (most recent call last):

  File "C:\Users\Nanni\git\odoo\openerp\netsvc.py", line 300, in dispatch_rpc

    result = ExportService.getService(service_name).dispatch(method, params)

  File "C:\Users\Nanni\git\odoo\openerp\service\web_services.py", line 632, in dispatch

    res = fn(db, uid, *params)

  File "C:\Users\Nanni\git\odoo\openerp\osv\osv.py", line 190, in execute_kw

    return self.execute(db, uid, obj, method, *args, **kw or {})

  File "C:\Users\Nanni\git\odoo\openerp\osv\osv.py", line 132, in wrapper

    return f(self, dbname, *args, **kwargs)

  File "C:\Users\Nanni\git\odoo\openerp\osv\osv.py", line 199, in execute

    res = self.execute_cr(cr, uid, obj, method, *args, **kw)

  File "C:\Users\Nanni\git\odoo\openerp\osv\osv.py", line 187, in execute_cr

    return getattr(object, method)(cr, uid, *args, **kw)

  File "C:\Users\Nanni\git\odoo\addons\purchase\wizard\purchase_line_invoice.py", line 99, in makeInvoices

    acc_id = purchase_obj._choose_account_from_po_line(cr, uid, line, context=context)

  File "C:\Users\Nanni\git\odoo\addons\purchase\purchase.py", line 502, in _choose_account_from_po_line

    acc_id = property_obj.get(cr, uid, 'property_account_expense_categ', 'product.category', context=context).id

AttributeError: 'bool' object has no attribute 'id'

2015-09-24 15:14:23,752 4952 INFO TOSA_DEBUG werkzeug: 127.0.0.1 - - [24/Sep/2015 15:14:23] "POST /web/dataset/call_button HTTP/1.1" 200 -

2015-09-24 15:14:30,742 4952 ERROR TOSA_DEBUG openerp.osv.osv: Uncaught exception

Traceback (most recent call last):

  File "C:\Users\Nanni\git\odoo\openerp\osv\osv.py", line 132, in wrapper

    return f(self, dbname, *args, **kwargs)

  File "C:\Users\Nanni\git\odoo\openerp\osv\osv.py", line 199, in execute

    res = self.execute_cr(cr, uid, obj, method, *args, **kw)

  File "C:\Users\Nanni\git\odoo\openerp\osv\osv.py", line 187, in execute_cr

    return getattr(object, method)(cr, uid, *args, **kw)

  File "C:\Users\Nanni\git\odoo\addons\purchase\wizard\purchase_line_invoice.py", line 99, in makeInvoices

    acc_id = purchase_obj._choose_account_from_po_line(cr, uid, line, context=context)

  File "C:\Users\Nanni\git\odoo\addons\purchase\purchase.py", line 502, in _choose_account_from_po_line

    acc_id = property_obj.get(cr, uid, 'property_account_expense_categ', 'product.category', context=context).id

AttributeError: 'bool' object has no attribute 'id'

2015-09-24 15:14:30,742 4952 ERROR TOSA_DEBUG openerp.netsvc: 'bool' object has no attribute 'id'

Traceback (most recent call last):

  File "C:\Users\Nanni\git\odoo\openerp\netsvc.py", line 300, in dispatch_rpc

    result = ExportService.getService(service_name).dispatch(method, params)

  File "C:\Users\Nanni\git\odoo\openerp\service\web_services.py", line 632, in dispatch

    res = fn(db, uid, *params)

  File "C:\Users\Nanni\git\odoo\openerp\osv\osv.py", line 190, in execute_kw

    return self.execute(db, uid, obj, method, *args, **kw or {})

  File "C:\Users\Nanni\git\odoo\openerp\osv\osv.py", line 132, in wrapper

    return f(self, dbname, *args, **kwargs)

  File "C:\Users\Nanni\git\odoo\openerp\osv\osv.py", line 199, in execute

    res = self.execute_cr(cr, uid, obj, method, *args, **kw)

  File "C:\Users\Nanni\git\odoo\openerp\osv\osv.py", line 187, in execute_cr

    return getattr(object, method)(cr, uid, *args, **kw)

  File "C:\Users\Nanni\git\odoo\addons\purchase\wizard\purchase_line_invoice.py", line 99, in makeInvoices

    acc_id = purchase_obj._choose_account_from_po_line(cr, uid, line, context=context)

  File "C:\Users\Nanni\git\odoo\addons\purchase\purchase.py", line 502, in _choose_account_from_po_line

    acc_id = property_obj.get(cr, uid, 'property_account_expense_categ', 'product.category', context=context).id

AttributeError: 'bool' object has no attribute 'id'