Bug: delimiter disappears if any block is added after it

When you add a delimiter to the editor and try to add any block after it, the delimiter disappears for some reason.

#### Steps to reproduce
1. Add a delimiter block to the editor
2. Activate the "Add" tool on the delimiter, and select any block.

The demonstration below was recorded on the Editor.js demo located on [https://editorjs.io/](https://editorjs.io/).