Fix the exception when export saving order is less than 3 in export tab of preferences.

[papatekken]

When saving the Preferences, if no. of items in export tab->saving order list is less than 3, exception is raised.

So in this change, it will check the no. of items in saving order list before save to corresponding prerferences attribute.

That will fix #10157

Mandatory checks

• Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.

[Siedlerchr]

Thanks for your contribution! Overall looks good, just the changelog and then it's good from my side

[koppor]

Tried out locally. Works well.

Nitpick remaining ^^.

[Siedlerchr]

Thanks for the update! looks good

[calixtus]

Does not look very beautiful, but I guess it's the correct and there is no nicer way to do this. Therefor looks 'good' to me 😄

[papatekken]

Does not look very beautiful, but I guess it's the correct and there is no nicer way to do this. Therefor looks 'good' to me 😄

I agree it does not look very nice.
To do it in a better way it may need a bigger change with the data structure for the saving order preference, and the current preference export file need change too.