The user-entered string '' must begin with a '/' or '?'

Hi,

Firstly: Thank you for the plugin - it's great!

Repeatable: Always
Steps to repeat:
1. Install page_manager and page_manager_ui
2. Setup a page
3. Set the path to "/"
4. Visit /admin/structure/page_manager directly.
5. System error if debugging is not turned on.

Modules enabled:
- page_manager
- page_manager_ui
- panels

Expected Results:
Obviously my take away from this was that path shouldn't be set to "/" , however, maybe a error during the UI step would work best?

Actual Results:

InvalidArgumentException: The user-entered string '' must begin with a '/', '?', or '#'. in Drupal\Core\Url::fromUserInput() (line 204 of core/lib/Drupal/Core/Url.php).
Drupal\page_manager_ui\Entity\PageListBuilder->getPath(Object) (Line: 37)
Drupal\page_manager_ui\Entity\PageListBuilder->buildRow(Object) (Line: 229)
Drupal\Core\Entity\EntityListBuilder->render() (Line: 83)
Drupal\page_manager_ui\Entity\PageListBuilder->render() (Line: 22)
Drupal\Core\Entity\Controller\EntityListController->listing('page')
call_user_func_array(Array, Array) (Line: 123)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 574)
Drupal\Core\Render\Renderer->executeInRenderContext(Object, Object) (Line: 124)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext(Array, Array) (Line: 97)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}()
call_user_func_array(Object, Array) (Line: 144)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 64)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1) (Line: 57)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1) (Line: 99)
Drupal\page_cache\StackMiddleware\PageCache->pass(Object, 1, 1) (Line: 78)
Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1) (Line: 50)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1) (Line: 23)
Stack\StackedHttpKernel->handle(Object, 1, 1) (Line: 656)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)

Once again - thanks for the great work.