To reproduce:
Make a site that has Content Type A with an entity reference field to Content Type B
Make some content that has this reference as a sample
Make a view that shows content of Type A
Add an exposed grouped filter on Content Type A's ER field to Content Type B
Fill in a few examples of Content type B as options for the grouped filter. Use the Type B nid as the value.
Save and look at the view on a page. You should see the following:
"Notice: Undefined index: value in Drupal\views\Plugin\views\filter\NumericFilter->acceptExposedInput()"