Fixed: Value must be a user instance

[bsekachev]

Motivation and context

Resolved #2440

How has this been tested?

Manual testing

Checklist

• I submit my changes into the develop branch
• I have added description of my changes into CHANGELOG file
• I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues (read 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.
• I have updated the license header for each file (see an example below)

# Copyright (C) 2020 Intel Corporation
#
# SPDX-License-Identifier: MIT

[bsekachev]

@dvkruchinin Could you please write a test for the issue? The scenario is described in #2240. It is expected that UI starts fetching (loading circle) after changing assignee (in the case of exception it doesn't occur). It might be an anchor during testing or you can check console output somehow.

[dvkruchinin]

It is expected that UI starts fetching (loading circle) after changing assignee (in the case of exception it doesn't occur). It might be an anchor during testing or you can check console output somehow.

Sure. The test will be written.

[coveralls]

Coverage decreased (-0.02%) to 62.811% when pulling 5bfe7cb on bs/fixed_issue_2440 into a9b2817 on develop.