Fix Invalid Bibtexkey freeze

[grimes2]

Fixes #1882. Reference: #1863.

• Change in CHANGELOG.md described
• Manually tested changed features in running JabRef

[grimes2]

Please add label "ready-for-review". Thanks.

[Siedlerchr]

LGTM 👍 I tested your changes and they are working. Good work!
Please resolve the merge conflicts and then we can merge it it.

To resolve the merge conflicts I recommend installing and setting up a merge tool for 3way merge (e.g. Meld or kdiff3 or your favorite tool)
Then you do
git checkout master
git fetch upstream
git merge upstream/master
git push
git merge upstream/<yourBranch>
Open Git Gui to see and resolve merge conflicts
Be careful with the changelog,

[Siedlerchr]

Well done 👍 Merged.