[Bug Report] table component expands row function to report props error

[Pentium286]

Element UI version

2.6.3

OS/Browsers version

macOS 10.14.5/chrome 74.0.3729.169

Vue version

2.5.13

Reproduction Link

https://codepen.io/pen/?&editable=true

Steps to reproduce

就是把官方的例子复制到项目里，就报错了

[Vue warn]: Property or method "props" is not defined on the instance but referenced during render. Make sure that this property is reactive, either in the data option, or for class-based components, by initializing the property. See: https://vuejs.org/v2/guide/reactivity.html#Declaring-Reactive-Properties.

What is Expected?

可以正常显示

What is actually happening?

报错，无法显示table包括展开项

[element-bot]

Translation of this issue:

Element UI version

2.6.3

OS/Browsers version

MacOS 10.14.5/chrome 74.0.3729.169

Vue version

2.5.13

Reproduction Link

Https://codepen.io/pen/?&editable=true

Steps to reproduce

If you copy the official example into the project, you will make a mistake.

` '.
[Vue warn]: Property or method "props" is not defined on the instance but referenced during render. Make sure that this property is reactive, either in the data option, or for class-based components, by initializing the property. See: https://vuejs.org/v2/guide/reactivity.html#Declaring-Reactive-Properties.

` '.

What is Expected?

It can be displayed normally.

What is actually happening?

Error, unable to display table including expansion items

[ziyoung]

Issues without a valid reproduction link will be closed directly.