Check for NULLs when getting the 'next' message

[pedro-w]

Description

Check for NULLs which otherwise would cause the REPL to crash with a segmentation fault

Type of Change

• 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 not work as expected)
• Documentation update
• Performance improvement
• Code refactoring

Related Issues

Fixes #477

Changes Made

Add null checks before dereferencing

Testing

• All existing tests pass
• Added new tests for the changes
• Tested on the following platforms:
  • Linux
  • macOS (Intel)
  • macOS (Apple Silicon)
  • Windows
  • Other:

Test Commands

Io> a := list(1, 2, 3, 4) ==> list(1, 2, 3, 4) Io> a at(0) ==> 1 Io> a at(0) = 1 

(crashes without this PR)

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my own code
• 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
• Any dependent changes have been merged and published

Screenshots (if applicable)

Additional Notes

[stevedekorte]

Looks good - thanks!

[stevedekorte]

Thanks Pedro!