Fixed fulltext fetcher button invisible

[AnandChandrakar]

Closes #10425

Fixes

fixed an issue where the fulltext search button in entry editor used to disappear on click till the search is completed.

Screenshots

Now shows progress animation as it should when search is in progress as it should.

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.

[calixtus]

Looks good to me. Thanks!