fix(TreeData): Reset the childrens prop when unflattening dataset in case it is being reused

[jano-kucera]

fixes #1657 again

Recreate the children array from scratch everytime instead of entity lookups.

There were some lint errors which I had to fix to be able to commit this.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.8%. Comparing base (7a6dd06) to head (a842223).
Report is 8 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #1675     +/-   ##
========================================
- Coverage    99.8%   99.8%   -0.0%     
========================================
  Files         187     187             
  Lines       31082   31080      -2     
  Branches     9786    9783      -3     
========================================
- Hits        30993   30991      -2     
  Misses         89      89             

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[ghiscoding]

This is now release, but I even found another huge perf improvement via PR #1681 but of course I found it after I had already released everything lol 😆 So I'll wait a little before releasing this last perf, everything else is released. Thanks for the contribution. Cheers ⭐