Bug report

Since 1.8.3 we are getting an error when trying to access array both with [] and adding items on nested property ['_labels'][].

The code is located here: https://github.com/sonata-project/SonataAdminBundle/blob/4.x/src/Form/DataTransformer/ModelToIdPropertyTransformer.php#L194-L195

Code snippet that reproduces the problem

https://phpstan.org/r/6a9d7024-7818-4de5-b224-961eebcda927

Expected output

No errors

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