fix: not crash on string value

[zombieJ]

No description provided.

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[codecov]

Codecov Report

Merging #251 (6d9345f) into master (a15810e) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #251   +/-   ##
=======================================
  Coverage   99.79%   99.79%           
=======================================
  Files          15       15           
  Lines         485      485           
  Branches      123      122    -1     
=======================================
  Hits          484      484           
  Misses          1        1           

Impacted Files	Coverage Δ
src/Cascader.tsx	100.00% <100.00%> (ø)

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[zombieJ]

+ [email protected]

[afc163]

是不是也解决了 #221 的问题