Fix summary for closed file

[jishnub]

Currently, we assert that the file is open in summary before reading fields from it. However, this obviously fails for a closed file. In this PR, we add a branch for a closed file, and change the assert to a check.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.30%. Comparing base (f380e2c) to head (f6ce2d6).
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #225      +/-   ##
==========================================
+ Coverage   91.29%   91.30%   +0.01%     
==========================================
  Files           5        5              
  Lines         689      690       +1     
==========================================
+ Hits          629      630       +1     
  Misses         60       60              

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