When Image Variation has an identifier starting with a number, an exception will be thrown inside Admin UI when retrieving SubItems.

Steps to reproduce
1. Clean installation of Ibexa DXP v3.3
2. Create a new custom image variation:

    system:
        default:
            image_variations:
                50_square:
                    reference: null
                    filters:
                        geometry/scaleexact: [50, 50]

3. In CLI run:

php bin/console ibexa:graphql:generate-schema
php bin/console cache:clear

4. Login into the back office as an admin user and go to "Content structure".

Result
A "Cannot load location" error will be displayed and in logs, there will be an exception that says:

Names must match /^[_a-zA-Z][_a-zA-Z0-9]*$/ but "50_square" does not.

and no item will be rendered inside the "Sub-items" section.

Expected result
No error is presented and items in the "Sub-items" section are displayed normally.