Fix babs merge error when deleting merge_ds

[tien-tong]

Closes #323.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 84.21053% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.89%. Comparing base (81d3f7a) to head (fc802a9).

Files with missing lines	Patch %	Lines
babs/merge.py	84.21%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #322      +/-   ##
==========================================
+ Coverage   78.55%   78.89%   +0.33%     
==========================================
  Files          16       16              
  Lines        1735     1772      +37     
  Branches      288      293       +5     
==========================================
+ Hits         1363     1398      +35     
- Misses        260      264       +4     
+ Partials      112      110       -2     

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