fix(chips): null check for checkmark ref

[moog16]

fixes chips when filter chips are unmounted. setCheckmarkWidth ref method is called, but checkmark is null.

[williamernest]

LGTM

[codecov-io]

Codecov Report

Merging #677 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #677   +/-   ##
=======================================
  Coverage   94.83%   94.83%           
=======================================
  Files          67       67           
  Lines        2768     2768           
  Branches      414      414           
=======================================
  Hits         2625     2625           
  Misses         50       50           
  Partials       93       93

Impacted Files	Coverage Δ
packages/chips/ChipSet.tsx	98.64% <100%> (ø)	⬆️

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