It seems like $this->resource->model can also be of type Illuminate\Database\Eloquent\Model; however, parameter $model of Umbrellio\LTree\Resource...Resource::toTreeArray() does only seem to accept Umbrellio\LTree\Interfaces\LTreeInterface, maybe add an additional type check?
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Ignorable by Annotation
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If this is a false-positive, you can also ignore this issue in your code via the ignore-type annotation