fix: Assign event channel streams to null on stop()

[beroso]

Fix #1537

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 38.24%. Comparing base (a408310) to head (2b87c69).

Files with missing lines	Patch %	Lines
.../method_channel/mobile_scanner_method_channel.dart	0.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1538      +/-   ##
===========================================
- Coverage    38.32%   38.24%   -0.08%     
===========================================
  Files           42       42              
  Lines          989      991       +2     
===========================================
  Hits           379      379              
- Misses         610      612       +2     

Flag	Coverage Δ
unittests	38.24% <0.00%> (-0.08%)	⬇️

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[juliansteenbakker]

Looks good to me!

[navaronbracke]

@juliansteenbakker does this fix #1042 ?