Adding null check before clearing mouseDownTimeout

[nzayatz14]

Hello,

I was recently doing some performance testing on a virtual table component I had written. With rows constantly unmounting & remounting, I wanted to make sure the scroll was still smooth with tooltips (rc-tooltips to be exact).

During my tests, I noticed that a significant amount of my table cells needed to clear a timeout upon unmounting of the row, this caused lag in my scrolling.

I found the call was made by rc-trigger in it's componentWillUnmount function. I added a simple null-check before the clearTimeout function and it significantly improved my scrolling performance.

I also noticed that the other timer, this.delayTimer, has its own function that handles this case -> this.clearDelayTimer() and was wondering why this.mouseDownTimeout did not have something similar.

Thanks!
Nick

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.22%. Comparing base (b8b8cb2) to head (67c7afc).
⚠️ Report is 365 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #201   +/-   ##
=======================================
  Coverage   88.22%   88.22%           
=======================================
  Files          10       10           
  Lines         484      484           
  Branches      121      122    +1     
=======================================
  Hits          427      427           
  Misses         57       57           

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[shc023]

I was also running into perf issue caused by the same code when unmounting 10k+ rc-tooltip at the same time. I made a PR that does this and also clears the pointer itself after executing, which potentially gives even more performance gain:

#278