Account for null when checking for publishables

[wrgoto]

Adds check for nullables.

[codecov-io]

Codecov Report

Merging #38 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #38   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           4      4           
  Lines         109    109           
  Branches       21     21           
=====================================
  Hits          109    109

Impacted Files	Coverage Δ
src/components/subscribe.js	100% <100%> (ø)	⬆️

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