fix(core): unable to pass non initialized channels in StreamChannel

[xsahil03x]

Resolves: #2080 , #2096, #2113

Description of the pull request

• Fixed an issue with not being able to use a non-initialized Channel in the StreamChannel widget.
• Fixed issues with message action modals background getting covered by Material background color.

[codecov]

Codecov Report

Attention: Patch coverage is 84.61538% with 6 lines in your changes missing coverage. Please review.

Project coverage is 60.57%. Comparing base (d16844a) to head (5229364).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
...ream_chat_flutter_core/lib/src/stream_channel.dart	84.61%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2157      +/-   ##
==========================================
+ Coverage   60.55%   60.57%   +0.01%     
==========================================
  Files         383      383              
  Lines       23932    23927       -5     
==========================================
  Hits        14493    14493              
+ Misses       9439     9434       -5     

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[Brazol]

LGTM