fix: account for this.popup being potentially undefined in completion mousewheel listener

[akoreman]

Issue #, if available: NA

Description of changes: When using completion, a mousewheel listener is attached to close the popup on scroll. In some circumstances, this.popup can be undefined causing errors to be thrown and scrolling to break. This change accounts for this.popup being possibly undefined here.

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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 0% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 86.64%. Comparing base (7869158) to head (01abcfa).

Files	Patch %	Lines
src/autocomplete.js	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5537   +/-   ##
=======================================
  Coverage   86.64%   86.64%           
=======================================
  Files         593      593           
  Lines       42839    42839           
  Branches     7120     7120           
=======================================
  Hits        37117    37117           
  Misses       5722     5722           

Flag	Coverage Δ
unittests	86.64% <0.00%> (ø)

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