fix: crash in custom importers with worker threads

[alexander-akait]

This PR contains a:

• bugfix
• new feature
• code refactor
• test update
• typo fix
• metadata update

Motivation / Use-Case

fixes #955 fixes #956

Breaking Changes

No

Additional Info

No

[codecov]

Codecov Report

Merging #958 (7aeae05) into master (2b6b4f3) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #958   +/-   ##
=======================================
  Coverage   96.28%   96.28%           
=======================================
  Files           4        4           
  Lines         242      242           
  Branches       76       76           
=======================================
  Hits          233      233           
  Misses          8        8           
  Partials        1        1           

Impacted Files	Coverage Δ
src/utils.js	95.50% <100.00%> (ø)

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