fix: read property getBoundingClientRect of null when unmount

[lich-yoo]

经调查，出现在离开页面，组件卸载且页面出现滚动重置时，getOffset 参数会为 null

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[lich-yoo]

@zombieJ

[codecov]

Codecov Report

Merging #659 (676dd64) into master (730d4df) will decrease coverage by 0.11%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master     #659      +/-   ##
==========================================
- Coverage   99.65%   99.54%   -0.12%     
==========================================
  Files          28       28              
  Lines         876      878       +2     
  Branches      260      251       -9     
==========================================
+ Hits          873      874       +1     
- Misses          3        4       +1     

Impacted Files	Coverage Δ
src/stickyScrollBar.tsx	97.22% <50.00%> (-1.35%)	⬇️

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[afc163]

Could you add test case?