Error: Call to a member function toLink() on null

Problem/Motivation

Attempt to create a new order inside a Group results in fatal error.

Steps to reproduce

1. On clean Drupal install:
commerce_demo 2.x branch
commerceg_b2b
commerceg_b2b_order

2. Enable "default" and "physical" order types for Group
3. on the page: http://commerce.site/group/2/orders
in the block "Group operations" choose "Create default"
4. Get fatal error

Stack trace:

Error: Call to a member function toLink() on null in Drupal\commerce_order\Form\OrderForm->form() (line 115 of /var/www/html/modules/contrib/commerce/modules/order/src/Form/OrderForm.php)

#0 /var/www/html/core/lib/Drupal/Core/Entity/EntityForm.php(106): Drupal\commerce_order\Form\OrderForm->form(Array, Object(Drupal\Core\Form\FormState))
#1 [internal function]: Drupal\Core\Entity\EntityForm->buildForm(Array, Object(Drupal\Core\Form\FormState))
#2 /var/www/html/core/lib/Drupal/Core/Form/FormBuilder.php(532): call_user_func_array(Array, Array)
#3 /var/www/html/core/lib/Drupal/Core/Form/FormBuilder.php(278): Drupal\Core\Form\FormBuilder->retrieveForm('commerce_order_...', Object(Drupal\Core\Form\FormState))
#4 /var/www/html/core/lib/Drupal/Core/Entity/EntityFormBuilder.php(48): Drupal\Core\Form\FormBuilder->buildForm(Object(Drupal\commerce_order\Form\OrderForm), Object(Drupal\Core\Form\FormState))
#5 /var/www/html/modules/contrib/group/src/Entity/Controller/GroupContentController.php(361): Drupal\Core\Entity\EntityFormBuilder->getForm(Object(Drupal\commerce_order\Entity\Order), 'add', Array)
#6 [internal function]: Drupal\group\Entity\Controller\GroupContentController->createForm(Object(Drupal\group\Entity\Group), 'commerceg_order...')
#7 /var/www/html/core/lib/Drupal/Core/EventSubscriber/EarlyRenderingControllerWrapperSubscriber.php(123): call_user_func_array(Array, Array)
#8 /var/www/html/core/lib/Drupal/Core/Render/Renderer.php(573): Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}()
#9 /var/www/html/core/lib/Drupal/Core/EventSubscriber/EarlyRenderingControllerWrapperSubscriber.php(124): Drupal\Core\Render\Renderer->executeInRenderContext(Object(Drupal\Core\Render\RenderContext), Object(Closure))
#10 /var/www/html/core/lib/Drupal/Core/EventSubscriber/EarlyRenderingControllerWrapperSubscriber.php(97): Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext(Array, Array)
#11 /var/www/html/vendor/symfony/http-kernel/HttpKernel.php(158): Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}()
#12 /var/www/html/vendor/symfony/http-kernel/HttpKernel.php(80): Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object(Symfony\Component\HttpFoundation\Request), 1)
#13 /var/www/html/core/lib/Drupal/Core/StackMiddleware/Session.php(57): Symfony\Component\HttpKernel\HttpKernel->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#14 /var/www/html/core/lib/Drupal/Core/StackMiddleware/KernelPreHandle.php(47): Drupal\Core\StackMiddleware\Session->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#15 /var/www/html/core/modules/page_cache/src/StackMiddleware/PageCache.php(106): Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#16 /var/www/html/core/modules/page_cache/src/StackMiddleware/PageCache.php(85): Drupal\page_cache\StackMiddleware\PageCache->pass(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#17 /var/www/html/core/lib/Drupal/Core/StackMiddleware/ReverseProxyMiddleware.php(47): Drupal\page_cache\StackMiddleware\PageCache->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#18 /var/www/html/core/lib/Drupal/Core/StackMiddleware/NegotiationMiddleware.php(52): Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#19 /var/www/html/vendor/stack/builder/src/Stack/StackedHttpKernel.php(23): Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#20 /var/www/html/core/lib/Drupal/Core/DrupalKernel.php(706): Stack\StackedHttpKernel->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#21 /var/www/html/index.php(19): Drupal\Core\DrupalKernel->handle(Object(Symfony\Component\HttpFoundation\Request))
#22 {main}