Fix for running without optional packages

[marksweb]

No description provided.

[codecov-commenter]

Codecov Report

Merging #515 (d5d5831) into master (aa02dea) will increase coverage by 0.27%.
The diff coverage is 96.29%.

@@            Coverage Diff             @@
##           master     #515      +/-   ##
==========================================
+ Coverage   96.17%   96.44%   +0.27%     
==========================================
  Files          42       42              
  Lines        2171     2196      +25     
==========================================
+ Hits         2088     2118      +30     
+ Misses         83       78       -5     

Impacted Files	Coverage Δ
explorer/utils.py	88.88% <85.71%> (+0.55%)	⬆️
explorer/tests/test_exporters.py	100.00% <100.00%> (ø)
explorer/tests/test_models.py	100.00% <100.00%> (ø)
explorer/tests/test_schema.py	100.00% <100.00%> (ø)
explorer/tests/test_tasks.py	100.00% <100.00%> (ø)
explorer/tests/test_views.py	100.00% <100.00%> (ø)
explorer/tasks.py	95.77% <0.00%> (+4.22%)	⬆️
explorer/app_settings.py	100.00% <0.00%> (+5.40%)	⬆️

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