[Bug Report] MessageBox报错 TypeError: Cannot read property 'state' of undefined

### Mint UI version
2.2.13

### OS/Browsers version
mac os 10.13.6 (17G65) / chrome版本 69.0.3497.100

### Vue version
2.5.17

### Reproduction Link
[https://jsfiddle.net/mmx38qxw/6201/](https://jsfiddle.net/mmx38qxw/6201/)

### Steps to reproduce
直接点按钮 控制台就能看见错误
TypeError: Cannot read property 'state' of undefined
at VueComponent.mappedState

### What is Expected?
不报错

### What is actually happening?
报错