Fixed crash related to issue tries to mount to not existing parent

[bsekachev]

Motivation and context

Resolved #6950

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 #6977 (fc81fde) into develop (e699848) will increase coverage by 0.04%.
Report is 20 commits behind head on develop.
The diff coverage is 92.85%.

@@             Coverage Diff             @@
##           develop    #6977      +/-   ##
===========================================
+ Coverage    82.58%   82.63%   +0.04%     
===========================================
  Files          360      360              
  Lines        38914    38920       +6     
  Branches      3570     3570              
===========================================
+ Hits         32137    32161      +24     
+ Misses        6777     6759      -18     

Components	Coverage Δ
cvat-ui	77.69% <92.85%> (+0.08%)	⬆️
cvat-server	87.16% <ø> (+0.01%)	⬆️

[klakhov]

Need to add a line to changelog