Fix PluginChecker async issue

[dmitryd20]

Что сделано?

Добавлена проверка delegate != nil в PluginChecker

Зачем это сделано?

Ссылка на delegate слабая, а checkPlugins() вызывается асинхронно. Поэтому иногда получалось, что delegate удалялся до завершения checkPlugins и получался AssertionFailure.

На что обратить внимание?

• Продублировал проверку в CollectionPluginsChecker и TablePluginsChecker, так как поведение у них одинаковое
• Под этот фикс создана ветка release/7.3.9, ПР направлен в неё

Resolves #255

[github-actions[bot]]

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Generated by :no_entry_sign: Danger Swift against 45c95a051e9daf89ca96c57c020e3584bfee60db

[codecov-commenter]

Codecov Report

:exclamation: No coverage uploaded for pull request base (release/7.3.9@9d807df). Click here to learn what that means. Patch has no changes to coverable lines.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## release/7.3.9 #256 +/- ## ================================================ Coverage ? 63.20% ================================================ Files ? 135 Lines ? 5030 Branches ? 2265 ================================================ Hits ? 3179 Misses ? 1727 Partials ? 124 ``` | [Flag](https://app.codecov.io/gh/surfstudio/ReactiveDataDisplayManager/pull/256/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=surfstudio) | Coverage Δ | | |---|---|---| | [uitests](https://app.codecov.io/gh/surfstudio/ReactiveDataDisplayManager/pull/256/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=surfstudio) | `46.14% <0.00%> (?)` | | | [unittests](https://app.codecov.io/gh/surfstudio/ReactiveDataDisplayManager/pull/256/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=surfstudio) | `38.38% <0.00%> (?)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=surfstudio#carryforward-flags-in-the-pull-request-comment) to find out more.

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