Fix error on schema fields editor when using system fields in tab sub widgets

Steps to reproduce:

• create a tab widget
• add a container widget in a slot
• in this container, add another container
• again
• here in palette "widgets by property", pick the "system" schema and drag & drop "current lifecycle state" widget (for instance)

=> the following alert appears "No field found for schema 'system' and fieldId 'currentLifeCycleState'"

Indeed the field editor presents all schemas except the system one.