API Platform version(s) affected: 3.4.* (looking at the code, could be also 4.* but not tested)

Description

Having a resource which has a sub resource with a ApiProperty's genId at false generates an error while using command api:openapi:export:

In SchemaFactory.php line 240:

  Notice: Indirect modification of overloaded element of ArrayObject has no effect

How to reproduce

That's the issue, I don't know how I ended up with this.
I tried to replicate this in a fresh APIP 4.1 distrib, but didn't manage to.
I didn't find a 3.4 distrib to test on a clean environment, so I tried to replicate on some professional project.

Investigating via step debug, it seems related to both sub resources and the presence of normalizationContext.
It's like the SchemaFactory generates a sub schema (v3.4 l231) of the sub resource that doesn't contains @id (due to serialization groups?) and then tries to delete it (v3.4 l240) but the @id is not existing then triggering the error.

Possible Solution

In the JsonSchema/SchemaFactory (v3.4 l240, v.4.0 l230, v4.1 l229), add in the condition before removing the @id part, a test of the key to delete:

if (
    isset($subSchema->getDefinitions()[$subDefinitionName])
    && isset($subSchema->getDefinitions()[$subDefinitionName]['properties']['@id'])
) {
    unset($subSchema->getDefinitions()[$subDefinitionName]['properties']['@id']);
}

I can do a PR to fix it, but as I wasn't enable to reproduce it outside of 3.4 I wanted to have some external feedback (cc @soyuka)

Additional Context

Full error:

Error thrown while running command "api:openapi:export --output='docs/swagger_docs.json'". Message: "Notice: Indirect modification of overloaded element of ArrayObject has no effect" ["exception" => ErrorException { …},"command" => "api:openapi:export --output='docs/swagger_docs.json'","message" => "Notice: Indirect modification of overloaded element of ArrayObject has no effect"]

In SchemaFactory.php line 240:

  Notice: Indirect modification of overloaded element of ArrayObject has no effect       

api:openapi:export [-y|--yaml] [-o|--output [OUTPUT]] [--spec-version [SPEC-VERSION]] [--api-gateway]

The quoted code was introduced by 99262dc, which focus on command api:json-schema:generate, but make api:openapi:export fail in the above situation.