Fix npe occurs during client graceful offline.

[CrazyHZM]

What is the purpose of the change

Brief changelog

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• Make sure there is a GitHub_issue field for the change (usually before you start working on it). Trivial changes like typos do not require a GitHub issue. Your pull request should address just this issue, without pulling in other changes - one PR resolves one issue.
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• Write necessary unit-test to verify your logic correction, more mock a little better when cross module dependency exist. If the new feature or significant change is committed, please remember to add sample in dubbo samples project.
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[sonarqubecloud]

SonarCloud Quality Gate failed.   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
1 Code Smell

75.0% Coverage
0.0% Duplication

[codecov-commenter]

Codecov Report

Merging #12190 (aa664c6) into 3.2 (e3600af) will decrease coverage by 0.40%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                3.2   #12190      +/-   ##
============================================
- Coverage     69.93%   69.54%   -0.40%     
+ Complexity      103        2     -101     
============================================
  Files          3405     1593    -1812     
  Lines        159372    65440   -93932     
  Branches      26318     9526   -16792     
============================================
- Hits         111457    45508   -65949     
+ Misses        38234    15542   -22692     
+ Partials       9681     4390    -5291     

see 1885 files with indirect coverage changes

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