I am trying to code a module to provide a markup cell in each sale order line for quotes (Odoo 10). But I consistently get an error while loading the quote/sale order. Any help is welcomed.

This is the view:

<?xml version="1.0"?>
<odoo>
    <record id="view_order_form_markup model" model="ir.ui.view">
        <field name="name">sale.order.form.markup</field>
        <field name="model">sale.order</field>
        <field name="inherit_id" ref="sale.view_order_form"/>
        <field name="arch" type="xml">
            <xpath expr='//field[@name="order_line"]/form/group/group/field[@name="price_unit"]' position="before">
                <label for="markup" string="Markup"/><field name="markup"/>
                <label for="markup_per" string="Markup (%)"/><field name="markup_per"/>
            </xpath>
            <xpath expr='//field[@name="order_line"]/tree/field[@name="price_unit"]' position="before">
                <label for="markup" string="Markup"/><field name="markup"/>
                <label for="markup_per" string="Markup (%)"/><field name="markup_per"/>
            </xpath>
        </field>
    </record>
</odoo>

And the model:

# -*- coding: utf-8 -*-

from odoo import api, fields, models
import odoo.addons.decimal_precision as dp

class SaleOrderLine(models.Model):
    _inherit = "sale.order.line"

    markup = fields.Float(compute='_compute_markup', digits=dp.get_precision('.2f%'), store=False, readonly=True)
    markup_per = fields.Float(compute='_compute_markup_per', digits=dp.get_precision('.2f%'), store=False, readonly=False)


    @api.depends('price_unit', 'purchase_price')
    def _compute_markup(self):
        for record in self:
            if record.price_unit and record.purchase_price:
                record.markup = record.price_unit - record.purchase_price

    @api.depends('price_unit', 'purchase_price')
    def _compute_markup_per(self):
        for record in self:
            if record.purchase_price > 0:
                record.markup_per = 100.0 * (record.price_unit - record.purchase_price) / record.purchase_price

    @api.onchange('markup_per')
    def _onchange_markup_per(self):
            if self.markup_per:
                if self.markup_per > -9999.99 and self.markup_per < 9999.99:
                    self.price_unit = self.purchase_price * (1 + self.markup_per / 100.0)

But I am getting this error when I open a quote/sales order:

Uncaught TypeError: Type is not a constructor
http://localhost:8369/web/content/365-ad86b81/web.assets_backend.js:2119
Traceback:
TypeError: Type is not a constructor
    at for_ (http://localhost:8369/web/content/365-ad86b81/web.assets_backend.js:2119:1208)
    at http://localhost:8369/web/content/365-ad86b81/web.assets_backend.js:2035:312
    at Function._.map._.collect (http://localhost:8369/web/content/319-32fb078/web.assets_common.js:13:270)
    at _.(anonymous function) [as map] (http://localhost:8369/web/content/319-32fb078/web.assets_common.js:69:526)
    at Class.setup_columns (http://localhost:8369/web/content/365-ad86b81/web.assets_backend.js:2035:261)
    at Class.<anonymous> (http://localhost:8369/web/content/365-ad86b81/web.assets_backend.js:2036:480)
    at http://localhost:8369/web/content/365-ad86b81/web.assets_backend.js:2099:11
    at Object.<anonymous> (http://localhost:8369/web/content/319-32fb078/web.assets_common.js:3202:136)
    at Object.<anonymous> (http://localhost:8369/web/content/319-32fb078/web.assets_common.js:547:681)
    at fire (http://localhost:8369/web/content/319-32fb078/web.assets_common.js:541:299)