fix: calling setMode just before destroy causes error reading getLength

[nlujjawal]

Issue #, if available:
#5681

Description of changes:
When the editor is destroyed the pending callback(due to some async network request) created with editor.getSession.setMode causes an error which is avoided by aborting the callback in case if editor is destroyed.

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Pull Request Checklist:

• No backwards incompatible changes were made to Ace's public interface which is defined through the main typings file (ace.d.ts) and its references:
  • https://github.com/ajaxorg/ace/blob/master/ace.d.ts
  • https://github.com/ajaxorg/ace/blob/master/ace-modes.d.ts
  • https://github.com/ajaxorg/ace/blob/master/ace-extensions.d.ts

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 87.01%. Comparing base (442680c) to head (bf4b461).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/edit_session.js	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5685      +/-   ##
==========================================
- Coverage   87.01%   87.01%   -0.01%     
==========================================
  Files         598      598              
  Lines       43683    43685       +2     
  Branches     7204     7205       +1     
==========================================
+ Hits        38012    38013       +1     
- Misses       5671     5672       +1     

Flag	Coverage Δ
unittests	87.01% <50.00%> (-0.01%)	⬇️

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[akoreman]

Is there a way we can add an automated test for this? mainly to reduce the risk of regression here in the future.