*: check if GetStore returns nil (#4347)

[ti-chi-bot]

This is an automated cherry-pick of #4347

close #4344

Signed-off-by: disksing i@disksing.com

What problem does this PR solve?

If we use remove tombstone, GetStore may return nil. We should check it.

What is changed and how it works?

Check List

Tests

• Unit test

Related changes

• Need to cherry-pick to the release branch

Release note

Fix panic issue after TiKV node scales in

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• nolouch
• rleungx

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[nolouch]

/merge

[ti-chi-bot]

@nolouch: It seems you want to merge this PR, I will help you trigger all the tests:

/run-all-tests

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[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 37639a4

[nolouch]

/run-unit-tests

[nolouch]

/run-unit-tests

[nolouch]

/run-unit-tests

[codecov]

Codecov Report

Merging #4381 (b8290bb) into release-5.2 (2129ee6) will decrease coverage by 0.11%.
The diff coverage is 33.33%.

@@               Coverage Diff               @@
##           release-5.2    #4381      +/-   ##
===============================================
- Coverage        74.86%   74.75%   -0.12%     
===============================================
  Files              249      249              
  Lines            25641    25653      +12     
===============================================
- Hits             19197    19176      -21     
- Misses            4770     4790      +20     
- Partials          1674     1687      +13     

Flag	Coverage Δ
unittests	74.75% <33.33%> (-0.12%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
server/schedule/region_scatterer.go	82.90% <0.00%> (-2.19%)	⬇️
server/schedule/checker/replica_strategy.go	88.37% <50.00%> (-4.13%)	⬇️
server/schedulers/shuffle_region.go	75.67% <50.00%> (-1.79%)	⬇️
server/statistics/store.go	90.59% <100.00%> (ø)
pkg/dashboard/adapter/manager.go	78.72% <0.00%> (-4.26%)	⬇️
server/tso/allocator_manager.go	67.11% <0.00%> (-2.01%)	⬇️
server/election/lease.go	94.91% <0.00%> (-1.70%)	⬇️
pkg/btree/btree.go	85.78% <0.00%> (-0.97%)	⬇️
server/cluster/cluster.go	82.24% <0.00%> (-0.84%)	⬇️
server/grpc_service.go	46.89% <0.00%> (-0.80%)	⬇️
... and 8 more

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