TypeError when init prompt with inputValue

code
`MessageBox.prompt('msg', 'title', { inputValue: 'sth' })`

got
`TypeError: Cannot read property 'classList' of undefined
at VueComponent.validate (eval at (app.js:936), :3208:23)
at VueComponent.boundFn [as validate] (eval at (app.js:636), :127:12)
at VueComponent.inputValue (eval at (app.js:936), :3221:14)
at Watcher.run (eval at (app.js:636), :2495:19)
at flushSchedulerQueue (eval at (app.js:636), :2275:13)
at Array.eval (eval at (app.js:636), :456:20)
at nextTickHandler (eval at (app.js:636), :405:16)
at `

Seems that `this.$refs.input` is not ready.