fix(hansbug): fix uncompitable problem with walk

[HansBug]

Fix problem with walk function in treevalue 1.2.2.

[HansBug]

It is now crashed in #4 .

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (release/0.2.1@1e922e0). Click here to learn what that means.
The diff coverage is n/a.

@@               Coverage Diff                @@
##             release/0.2.1       #5   +/-   ##
================================================
  Coverage                 ?   97.53%           
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  Files                    ?       32           
  Lines                    ?     1544           
  Branches                 ?        0           
================================================
  Hits                     ?     1506           
  Misses                   ?       38           
  Partials                 ?        0           

Flag	Coverage Δ
unittests	97.53% <0.00%> (?)

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