prevent barcode scanner plugin from being invoked if it is already open

[JGreenlee]

testing done:
On iPhone 6S, double-tapped "scan code" multiple trials and observed no errors. The scanner successfully opened on each trial

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.

Project coverage is 29.41%. Comparing base (03dd1f0) to head (d4986e9).
Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
www/js/onboarding/WelcomePage.tsx	0.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1186      +/-   ##
==========================================
- Coverage   29.44%   29.41%   -0.03%     
==========================================
  Files         122      122              
  Lines        4914     4919       +5     
  Branches     1124     1125       +1     
==========================================
  Hits         1447     1447              
- Misses       3465     3470       +5     
  Partials        2        2              

Flag	Coverage Δ
unit	29.41% <0.00%> (-0.03%)	⬇️

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Files with missing lines	Coverage Δ
www/js/onboarding/WelcomePage.tsx	0.00% <0.00%> (ø)