1326 validate invalid further reading link

[sumandari]

Please kindly see #1326 This is to address error 502 from the previous fix

[codecov[bot]]

Codecov Report

Merging #1347 (b258604) into develop (0fc574f) will increase coverage by 0.16%. The diff coverage is 69.23%.

@@             Coverage Diff             @@
##           develop    #1347      +/-   ##
===========================================
+ Coverage    63.76%   63.92%   +0.16%     
===========================================
  Files          180      180              
  Lines         9628     9617      -11     
  Branches       751      750       -1     
===========================================
+ Hits          6139     6148       +9     
+ Misses        3252     3228      -24     
- Partials       237      241       +4     

Impacted Files	Coverage Δ
django_project/lesson/urls.py	86.66% <ø> (ø)
django_project/lesson/views/further_reading.py	69.13% <60.00%> (+16.82%)	:arrow_up:
django_project/lesson/utilities.py	74.00% <100.00%> (+2.57%)	:arrow_up:
django_project/changes/utils/render_to_pdf.py	76.47% <0.00%> (+29.41%)	:arrow_up:

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[sumandari]

Please test it on: https://staging.changelog.kartoza.com/en/qgis/lessons/#introduction-qgis-1

[NyakudyaA]

LGTM @sumandari