fix: don't throw unhandled errors in TabstopManager when EditSession becomes undefined

[andredcoliveira]

Issue #, if available:

• N/A

Description of changes:

• editor.setSession seems to account for the possibility of session being falsy (e.g.: undefined, null). At the end of this method, the editor signals there was a change in the session
• When the session is changed, TabstopManager runs its detach method. This method was eventually expecting to use the session instance to remove markers, which is not possible when it is undefined

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[codecov]

Codecov Report

Patch coverage: 97.02% and project coverage change: -0.01 ⚠️

Comparison is base (6bebb4e) 87.13% compared to head (77792d5) 87.13%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5193      +/-   ##
==========================================
- Coverage   87.13%   87.13%   -0.01%     
==========================================
  Files         564      564              
  Lines       45021    45031      +10     
  Branches     6909     6908       -1     
==========================================
+ Hits        39229    39236       +7     
- Misses       5792     5795       +3     

Flag	Coverage Δ
unittests	87.13% <97.02%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/keyboard/hash_handler.js	87.41% <93.54%> (-1.02%)	⬇️
src/snippets.js	86.84% <97.54%> (-0.17%)	⬇️
src/snippets_test.js	99.58% <100.00%> (+0.01%)	⬆️

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[nightwing]

The issue seems to go deeper, because now if user expands a snippet, and then changes the session by changing tabs, the markers stay stuck in the old session, i think we need to keep both editor and session as properties, and either clear or restore tabstops when switching sessions.

[andredcoliveira]

Thanks for the additions, @nightwing! They look good to me, but I can't approve since I'm the author of the PR