Invariant Violation: "Example" has not been registered. This can happen if:
* Metro (the local dev server) is run from the wrong folder. Check if Metro is running, stop it and restart it in the current project.
* A module failed to load due to an error and `AppRegistry.registerComponent` wasn't called.
原因：return "react_navigaition_demo";引号里面返回的一定要是项目名，不能是 "Example"或者其他名称。