Fix last remaining bug in custom field saving

[rmunn]

Description

Follow-up to #1372. Fixes the one remaining bug that #1372 had missed.

Fixes #1248.

Type of Change

Only keep lines below that describe this change, then delete the rest.

• Bug fix (non-breaking change which fixes an issue)

Testing on your branch

• Create a new non-send/receive project (for speed)
• Load the TestLangProj.7z data into it (from the test/common directory)
• Add data to all the custom fields (except CustMultiPara because multi-paragraph fields are disabled in LF right now) of the first entry
• Also change the part of speech
• Click to another entry
• Click back to the first entry
• Verify that the data you added is still there
• Do a browser refresh (F5)
• Verify that the data you entered is still there
  • If this step fails, then the data made it into localStorage but did not actually make it into Mongo

Checklist

• I have performed a self-review of my own code
• I have reviewed the title/description of this PR which will be used as the squashed PR commit message
• I have commented my code, particularly in hard-to-understand areas
• I have added tests that prove my fix is effective or that my feature works

qa.languageforge.org testing

Reviewers: add/replace your name below and check the box to sign-off/attest the feature works as expected on qa.languageforge.org

• Chris Hirt (2022-04-04 15:20)
• Reviewer2 (YYYY-MM-DD HH:MM)

[github-actions]

Unit Test Results

    1 files      1 suites   12s ⏱️
373 tests 373 ✔️ 0 💤 0 ❌

Results for commit 385f00c.