[ISSUE #9128] Fix NPE when gRPC client ack message immediately after changing proxy

[qianye1001]

…changing proxy

Which Issue(s) This PR Fixes

Fixes #9128

Brief Description

How Did You Test This Change?

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 20.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 47.93%. Comparing base (f6249e5) to head (11bd640).
Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
...tmq/proxy/grpc/v2/consumer/AckMessageActivity.java	20.00%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             develop    #9129      +/-   ##
=============================================
- Coverage      48.02%   47.93%   -0.09%     
- Complexity     12050    12055       +5     
=============================================
  Files           1320     1321       +1     
  Lines          92846    93024     +178     
  Branches       11898    11926      +28     
=============================================
+ Hits           44588    44594       +6     
- Misses         42748    42922     +174     
+ Partials        5510     5508       -2     

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[lollipopjin]

LGTM