Fixed exception "Cannot read properties of null (reading 'plot')"

[bsekachev]

Motivation and context

Not affects users somehow, just want to get rid of extra logged exceptions.

How has this been tested?

Checklist

• I submit my changes into the develop branch
• I have created a changelog fragment
• I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues (see GitHub docs)
• I have increased versions of npm packages if it is necessary
  (cvat-canvas,
  cvat-core,
  cvat-data and
  cvat-ui)

License

• I submit my code changes under the same MIT License that covers the project.
  Feel free to contact the maintainers if that's a concern.

[codecov]

Codecov Report

Merging #7422 (54e6990) into develop (e4aa63f) will increase coverage by 0.03%.
Report is 1 commits behind head on develop.
The diff coverage is 100.00%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #7422      +/-   ##
===========================================
+ Coverage    83.43%   83.47%   +0.03%     
===========================================
  Files          373      373              
  Lines        39760    39761       +1     
  Branches      3720     3721       +1     
===========================================
+ Hits         33175    33190      +15     
+ Misses        6585     6571      -14     

Components	Coverage Δ
cvat-ui	79.36% <100.00%> (+0.07%)	⬆️
cvat-server	87.24% <ø> (ø)