Prerequisites - none

Steps:

1. Go to Admin > Content Types
2. Click button + to create content type group
3. Fill fields and save
4. Click button + to create content type
5. Fill necessary fields, add two fieldtypes
6. Click Discard changes
7. Go to Content Type Groups list
8. Select previously created group and delete it

Actual result:

Content type group is empty but we cannot delete it.

Argument '$contentTypeGroup' is invalid: Content type group has content type instances

Expected result:

Content type group is empty and we can delete it.