Clarify how null safety relates to runtime errors in variables.md

[matthew-a-thomas]

Replace overstatement about absence of all runtime errors with more precise language.

Surely, Dart apps can cause runtime errors 🙃

Instead, the wonderful benefit of null safety is the reduction of this specific kind of runtime error.

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• This PR doesn't contain automatically generated corrections or text (Grammarly, LLMs, and similar).
• This PR follows the Google Developer Documentation Style Guidelines — for example, it doesn't use i.e. or e.g., and it avoids I and we (first person).
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[gemini-code-assist]

Summary of Changes

Hello @matthew-a-thomas, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request improves the accuracy of the Dart language documentation by clarifying how null safety relates to runtime errors. It updates a section in variables.md to provide a more precise explanation of null safety's benefits, ensuring readers understand that it specifically targets null dereference errors rather than all possible runtime issues.

Highlights

• Documentation Clarification: Refined the language in variables.md to accurately describe the impact of null safety on runtime errors. The previous statement implying Dart apps cannot cause any runtime errors has been corrected to specify that null safety prevents null dereference runtime errors.

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[gemini-code-assist]

Code Review

This pull request makes a valuable correction to the documentation, removing an overstatement about runtime errors in Dart. The new wording is more precise. I've added one suggestion to further refine the language to be even more explicit about the benefits of null safety's compile-time checks.

[gemini-code-assist]

This is a great clarification that removes the previous overstatement. To make the benefit of null safety even clearer, you could consider rephrasing to explicitly mention that this check happens at compile time. This would reinforce the point made just before the example.

Suggested change

	In other languages, trying this could lead to a runtime error.
	Dart's compiler prevents this error by prohibiting these actions.
	In other languages, trying this could lead to a runtime error.
	With null safety, Dart reports this potential error at compile time.