Export PDF/HTML by command palette is not work

[Jocs]

Description

• Can you reproduce the issue?

Steps to reproduce

1. Shift + CmdOrCtrl + P to open the command palette
2. Input export
3. Select export PDF/HTML

Expected behavior:

Export file successfully.

Actual behavior:

vue.esm.js?a026:1897 Error: Invalid type to export: "undefined".
    at VueComponent.handleExport (VM647 editor.vue:1107)
    at invokeWithErrorHandling (vue.esm.js?a026:1863)
    at Vue.$emit (vue.esm.js?a026:3891)
    at execute (index.js?ce51:127)

Error happened.

Link to an example: [optional]

Versions

• Mark Text: develop
• Operating system: all