Here's a patch that fixes this issue.
samuel.seide → created an issue.
Here is a patch that should work for 8.3.5. See 1038316-131-field-multiple-remove-item-alt.patch if you're running 8.4.x
@MRPRAVIN Your patch contains a custom module in it - modules/custom/rb_account_profile/rb_account_profile.module b/modules/custom/rb_account_profile/rb_account_profile.module so there's no way to test it...
My expectation is that the patch should have some checking in it so that if the $item['_remove'] is not available then it won't run that code. Basically the patch should be able to function without it throwing up notices when a default paragraph is being used.
I tested the 8.4.x patch on a vanilla install of drupal 8.4.x dev, with the paragraphs and field_group module installed. If you create a field group add a entity reference paragraphs inside the group and give that a default paragraph, you will end up with this error:
in the code, the error is originating from the code $remove_element = $item['_remove']; as the default paragraph doesn't have a value for $item['_remove'] and so it comes over as null.
Here's how I had the paragraph setup in the content type: