CVAT new UI: Cannot read property 'annotations' of null

[azhavoro]

My actions before raising this issue

• Read/searched the docs
• Searched past issues

TypeError: Cannot read property 'annotations' of null

Expected Behaviour

N/A

Current Behaviour

Exception occurs

Possible Solution

Steps to Reproduce (for bugs)

N/A

Context

Your Environment

• Git hash commit (git log -1): 35badcc
• Docker version docker version: Docker version 19.03.8, build afacb8b7f0
• Are you using Docker Swarm or Kubernetes? No
• Operating System and version (e.g. Linux, Windows, MacOS): Ubuntu 18.04
• Code example or link to GitHub repo or gist to reproduce problem:
• Other diagnostic information / logs:
  JS stack trace:

TypeError: Cannot read property 'annotations' of null
    at Lo.beforeUnloadCallback (https://cvat.org/cvat-ui.min.js:72:1177773)

Next steps

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[bsekachev]

Steps to reproduce:

1. Open tool (DO NOT OPEN A JOB)
2. Go to settings
3. Close the tab

beforeUnloadCallback is called and tries to read property annotations of jobInstance that is null.
It will be resolved with #1705.