Fix undefined error during export

[jan-zajic]

Now sometimes (for some documents) during export Uncaught undefined is thrown:

C:\Program Files\PanWriter\resources\app.asar\src\js\Exporter.js:105 Uncaught (in promise) TypeError: Cannot read property 'output' of undefined
    at extractOut (C:\Program Files\PanWriter\resources\app.asar\src\js\Exporter.js:105)
    at mergeAndValidate (C:\Program Files\PanWriter\resources\app.asar\src\js\Exporter.js:109)
    at fileExport (C:\Program Files\PanWriter\resources\app.asar\src\js\Exporter.js:59)

this PR fix it. Thank you.

[mb21]

thanks!