Are you absolutely sure you are using react 16.0.0-alpha.12?
Check your package.json if you have a ^ before the react version, if you have, it probably have installed the latest react version, which currently is 16.0.0-alpha.13, in which it breaks as you say (just had the problem myself). Having the ^ before the version, allows it to install newer minor and patch versions. You can read more about it here.
To keep it at the exact version you specify, simply remove the ^ before the version, so that your package.json looks like this:
"dependencies": {
"react": "16.0.0-alpha.12",
"react-native": "0.45.1",
}
Remember to re-install your node_modules after your changes.