Fix TypeError when closing rendering

[sonelu]

Fixes #438

Description

Fixes TypeError exception that can appear when finishing with an environment that was rendered.

Fixes #438

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• I have run the pre-commit checks with pre-commit run --all-files (see CONTRIBUTING.md instructions to set it up)
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[Kallinteris-Andreas]

LGTM, from my testing it does fix the issue, and does not appear to break anything here or in Gymansium-Robotics