Front end dev problem: UnhandledPromiseRejectionWarning: Error: Exited with code 3

[tedjsmeng]

When I run "npm run dev" in the frontend folder, it responds error msg:

[email protected] dev /root/bilibili-vue
> node build/dev-server.js

(node:3989) UnhandledPromiseRejectionWarning: Error: Exited with code 3
    at ChildProcess.<anonymous> (/root/bilibili-vue/node_modules/opn/index.js:83:13)
    at Object.onceWrapper (events.js:300:26)
    at ChildProcess.emit (events.js:210:5)
    at maybeClose (internal/child_process.js:1021:16)
    at Socket.<anonymous> (internal/child_process.js:430:11)
    at Socket.emit (events.js:210:5)
    at Pipe.<anonymous> (net.js:658:12)
(node:3989) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 1)
(node:3989) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.

my nodejs is version 12.13.0