Only import gui widget when the toolkit is enabled

[ericpre]

Progress of the PR

• Only import gui widget when the toolkit is enabled to avoid unnecessary import of gui library,
• [n/a] update docstring (if appropriate),
• [n/a] update user guide (if appropriate),
• add an changelog entry in the upcoming_changes folder (see upcoming_changes/README.rst),
• Check formatting changelog entry in the readthedocs doc build of this PR (link in github checks)
• [n/a] add tests,
• ready for review.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.53%. Comparing base (de16955) to head (f226818).

Additional details and impacted files

@@                 Coverage Diff                 @@
##           RELEASE_next_minor    #3366   +/-   ##
===================================================
  Coverage               80.53%   80.53%           
===================================================
  Files                     146      146           
  Lines                   21864    21864           
  Branches                 5152     5152           
===================================================
  Hits                    17609    17609           
  Misses                   3037     3037           
  Partials                 1218     1218           

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