清空运行缓存就好了

* What went wrong:

Execution failed for task ':app:installDebug'.

> com.android.builder.testing.api.DeviceException: No online devices found.

* Try:

Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output. Run with --scan to get full insights.

* Get more help at https://help.gradle.org

BUILD FAILED in 2s

48 actionable tasks: 1 executed, 47 up-to-date

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 have

set up your Android development environment: