[ISSUE #8517] Fix client send UNREGISTER_CLIENT request twice may cause proxy NPE

[qianye1001]

Which Issue(s) This PR Fixes

Fixes #8517

Brief Description

client send UNREGISTER_CLIENT request twice may cause proxy NPE
check channel is null before use it, if channel is null, log warn instead of NPE

How Did You Test This Change?

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 16 lines in your changes missing coverage. Please review.

Project coverage is 46.86%. Comparing base (fa0cd49) to head (b3f7df8).

Files	Patch %	Lines
...proxy/remoting/activity/ClientManagerActivity.java	0.00%	16 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             develop    #8528      +/-   ##
=============================================
- Coverage      46.95%   46.86%   -0.09%     
+ Complexity     11325    11296      -29     
=============================================
  Files           1275     1275              
  Lines          89061    89067       +6     
  Branches       11454    11456       +2     
=============================================
- Hits           41815    41739      -76     
- Misses         42081    42155      +74     
- Partials        5165     5173       +8     

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