Fixed issue: Cannot read property label of undefined

[bsekachev]

Motivation and context

Resolve #1823

How has this been tested?

Manual testing
@dvkruchinin Could you please write a test for the issue?

Scenario is quite interesting, so, probably need to think how implement the test better.

1. First we need a task with at least two images and chunk size === 1 (there can be other conditions, but anyway during the test we need to fetch another chunk. In this simplest way it is achieved just by jumping to the next frame)
2. On the first frame need to create a shape
3. Next we need to go to another frame (chunk fetching procedure must be run [circle loading progress bar at the center of canvas, element #cvat_canvas_loading_circle])
4. During this procedure (duration depends on client-server bandwidth) need to open context menu for a shape
5. Before fix UI is expected to be crashed with this exception

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

[dvkruchinin]

Sure. The test will be written.

[coveralls]

Coverage decreased (-0.02%) to 62.811% when pulling c0d92ed on bs/fixed_issue_1823 into a9b2817 on develop.