fix: add null guard to toast._container

[thkruz]

Closes issue Dogfalo#1394

Proposed changes

Correct type for _container and add null guards.

Types of changes

• [ x] Bug fix (non-breaking change which fixes an issue).
• New feature (non-breaking change which adds functionality).
• Breaking change (fix or feature that would cause existing functionality to change).

Checklist:

• [ x] I have read the CONTRIBUTING document.
• [ x] My commit messages follows the conventional commit format
• My change requires a change to the documentation, and updated it accordingly.
• [ x] I have added tests to cover my changes.
• [ x] All new and existing tests passed.

[gselderslaghs]

Cool catch, works as expected
@wuda-io