`style.addSource` throws a `PlatformException` but `style.getSource` does not return the existing source

Nobody has claimed this yet.

I have

```
late final Source? source;

try {
source = await style.getSource(
'sourceId',
);
} catch (_) {
source = null;
}
```

Which returns a `null` on the ID `sourceId`. However when I try to add the source using the same ID `sourceId`
```
final geoJsonFeatureCollection = ...

final newSource = GeoJsonSource(
id: 'sourceId',
data: json.encode(
geoJsonFeatureCollection,
),
);

await style.addSource(
newSource,
);
```

I get a `PlatformException(0, Source sourceId already exists, null, null)`

I can't seem to get why this may be happening. Any pointers will be highly appreciated

PS: I am currently using version `2.3.0` of this library

Start by reproducing the behavior in the `style.getSource` and `style.addSource` calls using the reported `sourceId` and library version 2.3.0. Trace how each call handles existing sources and platform errors; done means the source lookup and add operation consistently reflect whether `sourceId` already exists, with a regression test if the repository provides a relevant test location.

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