Fix npe when closing a last reader that have not been used

[jt2594838]

No description provided.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 65.70%. Comparing base (7859080) to head (49d5a5a).
Report is 16 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #513   +/-   ##
========================================
  Coverage    65.70%   65.70%           
========================================
  Files          566      566           
  Lines        33360    33360           
  Branches      4664     4664           
========================================
  Hits         21919    21919           
  Misses       10789    10789           
  Partials       652      652           

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