[Bug Report] v-transfer-dom中的popup绑定v-model报错

### VUX version
2.9.4

### OS/Browsers version
win10-64位，360极速浏览器

### Vue version
2.6.10

### Code

```html

import { ViewBox, XHeader, Box, XButton, Tab, TabItem, XInput, Group, TransferDom, Popup} from 'vux'
export default {
directives: {
TransferDom
},
router,
data () {
return {
dialogShow: true
}
},
components: {
ViewBox,
XHeader,
XButton,
Box,
Tab,
TabItem,
Group,
XInput,
Popup
},
mounted: function () {
this.dialogShow = false
},
```

### Steps to reproduce
定义变量dialogShow为true,当dialogShow改变为false时，页面就会报错:
[Vue warn]: Error in callback for watcher "show": "TypeError: Cannot read property 'hide' of undefined"
found in
---> at F:\work\……\node_modules\_vux@2.9.4@vux\src\components\popup\index.vue

TypeError: Cannot read property 'hide' of undefined
at VueComponent.show

### What is Expected?
期望不报警告和错误

### What is actually happening?
[Vue warn]: Error in callback for watcher "show": "TypeError: Cannot read property 'hide' of undefined"
found in
---> at F:\work\……\node_modules\_vux@2.9.4@vux\src\components\popup\index.vue

TypeError: Cannot read property 'hide' of undefined
at VueComponent.show