Fix undefined volumes in customizable viewport overlay

[Sofien-Sellami]

Context

Fixing undefined volumes in _getViewportInstance function

Changes & Results

Testing

Checklist

PR

• My Pull Request title is descriptive, accurate and follows the
  semantic-release format and guidelines.

Code

• My code has been well-documented (function documentation, inline comments,
  etc.)

Public Documentation Updates

• The documentation page has been updated as necessary for any public API
  additions or removals.

Tested Environment

• OS:
• Node version:
• Browser:

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[codecov]

Codecov Report

Merging #3435 (d3dc760) into master (cadb559) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3435   +/-   ##
=======================================
  Coverage   38.27%   38.27%           
=======================================
  Files          82       82           
  Lines        1348     1348           
  Branches      303      303           
=======================================
  Hits          516      516           
  Misses        666      666           
  Partials      166      166           

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[salimkanoun]

I guess this one is for @sedghi, a simple fix for overlay that has a key error to retrieve data to display

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