Replace assert with proper null checks for content in write method

[Isurunwg]

Proposed changes

This PR introduces several important changes across multiple methods by replacing assert statements with proper runtime validation and handling.

Types of changes

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• Bugfix (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 (if none of the other choices apply)

Checklist

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• I have read the CONTRIBUTING.md
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

Further comments

This PR significantly enhances the runtime behavior by ensuring that important checks are always performed, even when assertions are disabled. The code is now more resilient to edge cases and potential errors and will give more informative feedback when things go wrong.