Editor field in fieldset field makes codemirrror css breaks

This `editor` field is not wrapped inside `fieldset`, no issue at all

This one is inside `fieldset` field, on first load, the editor is black and when you click inside the `code`appears but with css issues (`linenumbers`)

you can test with :
````
form:
fields:
general:
type: section
title: 'Image'
underline: true
fields:
header.test_editor:
type: editor
label: 'HTML'
autofocus: true
codemirror:
mode: 'text/html'
theme: 'monokai'
indentUnit: 4
autofocus: true
indentWithTabs: false
lineNumbers: true
styleActiveLine: true
gutters: ['CodeMirror-lint-markers']
lint: false
header.el_media_options:
type: fieldset
title: 'Image'
icon: tv
collapsed: true
collapsible: true
fields:
header.el_image_basic_svg_overlay_editor:
type: editor
label: 'HTML'
autofocus: true
codemirror:
mode: 'text/html'
theme: 'monokai'
indentUnit: 4
autofocus: true
indentWithTabs: false
lineNumbers: true
styleActiveLine: true
lint: false
````