Fix method that was not implementing Symfony Bridges DoctrineType abstract correctly

Composer.lock demonstrating the package versions I was working against:

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        {
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        {
            "package": "sensio/framework-extra-bundle",
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        {
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        {
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            "version": "dev-master",
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        },
        {
            "package": "symfony/swiftmailer-bundle",
            "version": "dev-master",
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        {
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