File picker gets stuck

[pedropintosilva]

Describe the bug

Hello,

as promised @juliusknorr here is the issue from our weekly meeting:

Review > Document Compare...

Triggers UI_InsertFile postmessage; Opens frozen filepicker “Insert file from Nextcloud”

https://github.com/nextcloud/richdocuments/blame/main/src/view/FilesAppIntegration.js#L179-L181 
vue.runtime.esm.js:3065 SyntaxError: "undefined" is not valid JSON
at JSON.parse (<anonymous>)
at Object.<anonymous> (FilesAppIntegration.js:178:41)
at FilePicker.vue:276:43
at i.Oe (FilePicker.vue:320:2)

[juliusknorr]

I'm unsure why this happens out of a sudden but we should guard the json parse with a check if the share attributes in https://github.com/nextcloud/richdocuments/blame/main/src/view/FilesAppIntegration.js#L179-L181 are actually set

[juliusknorr]

We should have a test covering inserting an image into the document.