fix type declaration of private fields in cvat-canvas which are possibly null

[tjdans1114]

Motivation and context

some private fields of classes used in cvat-canvas are type-declared as non-null, whereas they are actually assigned the null value.

How has this been tested?

Checklist

• I submit my changes into the develop branch
• I have added a 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.

[bsekachev]

Thank you for the contribution!