add editor.destroyed property

[nightwing]

• add editor.destroyed property
• keep $toDestroy as array even after destroying to prevent calling methods of destroyed editor from throwing errors
• prevent space scrolling editor out of view in readonly mode

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.25%. Comparing base (78ad9c7) to head (3d76701).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5785   +/-   ##
=======================================
  Coverage   87.24%   87.25%           
=======================================
  Files         604      604           
  Lines       44305    44309    +4     
  Branches     7265     7267    +2     
=======================================
+ Hits        38656    38660    +4     
  Misses       5649     5649           

Flag	Coverage Δ
unittests	87.25% <100.00%> (+<0.01%)	⬆️

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