Fix panic when ref is mis-used

[daveshanley]

Adds length check to prevent a panic if a $ref is mis-used.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.62%. Comparing base (6f277db) to head (66ffbcf).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #279   +/-   ##
=======================================
  Coverage   99.62%   99.62%           
=======================================
  Files         162      162           
  Lines       12085    12086    +1     
=======================================
+ Hits        12040    12041    +1     
  Misses         45       45           

Flag	Coverage Δ
unittests	99.62% <100.00%> (+<0.01%)	⬆️

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