[Bug Report] PopupPicker 遮罩层出错了

### VUX version
2.9.0

### OS/Browsers version
win10旗舰版 chrome浏览器55.0.2883.87 m

### Vue version
2.5.16

### Code

```html

import { Group, Checklist, XInput, PopupPicker, Picker } from 'vux'

export default {
name: 'applyGrade',
components: {
Group,
XInput,
Checklist,
PopupPicker,
Picker
},
data () {
return {
userName: '',
tel: '',
commonList: [ '种植户', '经纪人', '农资' ],
checklist001: [],
sexList: ['男', '女'],
checklist002: [],
list1: [['华为', '小米', 'oppo', 'vivo', '苹果']],
pickerValue: ['华为']
}
},
methods: {
hand () {
console.log('hand')
console.log('........', this.userName, this.tel, this.checklist001)
}
},
mounted () {
console.log('mounted')
}
}

body,div,dl,dt,dd,ul,ol,li,h1,h2,h3,h4,h5,h6,pre,code,form,fieldset,legend,input,textarea,p,blockquote,th,td { font-size: .32rem; padding: 0; margin: 0; }
.BIMain .BItop { position: relative; height: 1rem; line-height: 1rem; border-bottom: 1px solid #c1c1c1; margin-bottom: 5px; }
.BIMain .BItop p { width: 100%; text-align: center; }
.BIMain .iconLeft { position: absolute; top: 0; left: .32rem; height: 1rem; line-height: 1rem; }
.BIMain .BIinfo p { margin-bottom: 5px; }
.BIMain h3 { text-align: center; padding: .2rem 0; border-bottom: 1px solid #666; }
.BIMain .weui-label { text-align: center; }
.BIMain .btnClick { border: 0; outline: none; background: #339933; text-align: center; border-radius: 5px; display: inline-block; margin: 20px auto; width: 100%; height: 30px; line-height: 30px; }

```

### Steps to reproduce
去掉一个checklist，页面样式正常，2个checklist和一个PopupPicker在一起时，PopupPicker的遮罩层异常显示

### What is Expected?
PopupPicker初始化遮罩层没有显示

### What is actually happening?
PopupPicker的遮罩层在没有点击组件的情况下出现。如果此时再次点击组件取消遮罩，成功取消，然后滑动下页面，遮罩又显示出来，但此时遮罩是不应该显示的