React-Native之错误: 找不到或无法加载主类 org.

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错误信息：

Starting JS server...
Building and installing the app on the device (cd android && gradlew.bat
 installDebug)...错误: 找不到或无法加载主类 org.gradle.wrapper.GradleWrapperMain
Could not install the app on the device, read the error above for details.
Make sure you have an Android emulator running or a device connected and
 haveset up your Android development environment:
https://facebook.github.io/react-native/docs/android-setup.html

错误原因：

项目未创建完成，就运行了程序

解决办法：

删掉原项目，重新创建，再运行