Fixed an issue where entry preview tab has no name in drop down list.

[dextep]

Fixes #6591

I didn't add "Preview" to JabRef_en.properties because already exists.

• Change in CHANGELOG.md described (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 documentation: Is the information available and up to date? If not created an issue at https://github.com/JabRef/user-documentation/issues or, even better, submitted a pull request to the documentation repository.

[Siedlerchr]

Thanks for the quick fix!
I see you are using the dark theme. There are still some little issues in the dark theme. Maybe you are interested in taking a look at them?
#5522 (comment)