avoid access private variable

[sonichi]

Why are these changes needed?

Use a public method instead of access to a private variable.

Related issue number

#1575 #1402

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (d675489) 35.23% compared to head (b05cacc) 68.58%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1592       +/-   ##
===========================================
+ Coverage   35.23%   68.58%   +33.34%     
===========================================
  Files          44       44               
  Lines        5327     5331        +4     
  Branches     1236     1305       +69     
===========================================
+ Hits         1877     3656     +1779     
+ Misses       3296     1325     -1971     
- Partials      154      350      +196     

Flag	Coverage Δ
unittests	68.54% <100.00%> (+33.30%)	⬆️

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