[Bug Report] popup-picker为空时报错

### VUX version
2.9.2

### OS/Browsers version
win10；chrome版本 69.0.3497.100（正式版本） （64 位）

### Vue version
2.5.18-beta.0

### Code

```html

import { PopupPicker, Group, Cell, XInput } from 'vux'
import apis from '@/apis'
export default {
components: {PopupPicker, Group, Cell, XInput},
data () {
return {
targetStation: '',
busList: [],
bus: []
}
},
methods: {
getTargetStation (value) {
this.busList = []
this.bus = []
this.$http.post(apis.MyTrafficBusTimeByTargetStationFirstSearch, {targetStationName: value}, {
headers: {
'X-XSRF-TOKEN': 'N2fVeoHNdMVRDtBJVdPG9Xlg%2BWvq%2Fsy1hcWA1Bg3dfszK4feE5g4RpmG9uQqeYj5.b67cc358169a4670a90513e094792e5e'
}
}).then(res => {
if (res.data.status === '200') {
if (res.data.data.records.length > 0) {
this.busList = res.data.data.records.map(item => {
return {
name: item.routeName,
value: item.id
}
})
}
}
})
},
busChange (val) {
this.$http.post(apis.MyTrafficBusTimeByRouteNameSecondSearch, {routeId: val.join(',')}, {
headers: {
'X-XSRF-TOKEN': 'N2fVeoHNdMVRDtBJVdPG9Xlg%2BWvq%2Fsy1hcWA1Bg3dfszK4feE5g4RpmG9uQqeYj5.b67cc358169a4670a90513e094792e5e'
}
}).then(res => {
if (res.data.status === '200') {

}
})
}
}
}

```

### Steps to reproduce
页面刷新

### What is Expected?
不报错

### What is actually happening?
vue.esm.js?efeb:591 [Vue warn]: Error in nextTick: "TypeError: Cannot read property 'value' of undefined"

found in

---> at node_modules/vux/src/components/picker/index.vue
at node_modules/vux/src/components/popup/index.vue
at node_modules/vux/src/components/popup-picker/index.vue
at node_modules/vux/src/components/group/index.vue
at src/pages/myBus.vue
at src/App.vue

warn @ vue.esm.js?efeb:591
logError @ vue.esm.js?efeb:1737
globalHandleError @ vue.esm.js?efeb:1732
handleError @ vue.esm.js?efeb:1721
(anonymous) @ vue.esm.js?efeb:1839
flushCallbacks @ vue.esm.js?efeb:1758
Promise.then (async)
microTimerFunc @ vue.esm.js?efeb:1806
nextTick @ vue.esm.js?efeb:1850
Vue.$nextTick @ vue.esm.js?efeb:4517
(anonymous) @ vue-router.esm.js?fe87:1978
step @ vue-router.esm.js?fe87:1706
step @ vue-router.esm.js?fe87:1713
runQueue @ vue-router.esm.js?fe87:1717
(anonymous) @ vue-router.esm.js?fe87:1971
step @ vue-router.esm.js?fe87:1706
(anonymous) @ vue-router.esm.js?fe87:1710
(anonymous) @ vue-router.esm.js?fe87:1956
(anonymous) @ vue-router.esm.js?fe87:1784
iterator @ vue-router.esm.js?fe87:1935
step @ vue-router.esm.js?fe87:1709
step @ vue-router.esm.js?fe87:1713
runQueue @ vue-router.esm.js?fe87:1717
confirmTransition @ vue-router.esm.js?fe87:1964
transitionTo @ vue-router.esm.js?fe87:1866
init @ vue-router.esm.js?fe87:2494
beforeCreate @ vue-router.esm.js?fe87:539
callHook @ vue.esm.js?efeb:2921
Vue._init @ vue.esm.js?efeb:4626
Vue @ vue.esm.js?efeb:4729
(anonymous) @ main.js?1c90:16
./src/main.js @ app.js:5710
__webpack_require__ @ app.js:679
fn @ app.js:89
0 @ app.js:5799
__webpack_require__ @ app.js:679
(anonymous) @ app.js:725
(anonymous) @ app.js:728
vue.esm.js?efeb:1741 TypeError: Cannot read property 'value' of undefined
at VueComponent.render (index.vue?6c15:79)
at VueComponent.eval (index.vue?6c15:34)
at Array.eval (vue.esm.js?efeb:1837)
at flushCallbacks (vue.esm.js?efeb:1758)