Adding inline custom tags results in js error and makes users unable to edit them.

Steps to reproduce:
1. Create a new custom tag according to https://doc.ezplatform.com/en/latest/tutorials/extending_admin_ui/4_adding_a_custom_tag/#step-4-creating-a-custom-tag.
2. Make it inline by adding is_inline: true before attributes key in custom_tags.yaml configuration file.
3. Go to the BO and create a new article.
4. Try to add a newly configured inline custom tag.

Result:
You will be presented with a JS error and won't be able to edit it.

Uncaught TypeError: Cannot read property 'type' of null
    at ButtonCustomTagUpdate.saveCustomTag (ezplatform-richtext-onlineeditor-js.js:4527)Al

Also, the custom tag is created with unwanted <br/> inside and that results in an extra line rendered on the frontend.

Expected result:
The inline custom tag toolbar is responsive.

Post implementation note: Patch fixing this issue will cause console command `clear:cache` to fail with an exception indicating that inline custom tags in toolbars other than "text" are not allowed.

Note:
This issue doesn't affect custom tags without is_inline parameter.