When working with nullable/optional values or external/runtime-provided fields, never assume presence or a specific concrete type/spelling. Instead: (1) explicitly treat inputs as optional, (2) provide deterministic fallbacks (often empty string/None/previous-state), (3) avoid dereferencing missing values, and (4) add tests for both “value present” and “value absent / alternate field present” cases. Apply this especially to: - UI state: preserve the existing focus target if one exists (it may not be terminal input); don’t hardcode a single focus type. - Runtime metadata/env fields: probe multiple possible field names across versions and fall back safely when neither exists. - Generated values: when a field is unavailable, emit a safe inert default (e.g., `""`) rather than constructing partial commands/paths. Example (runtime field fallback): ```rust fn nu_home_path(env: &HashMap) -> String { // Prefer $nu.home-path when exposed; fall back to $nu.home-dir, then finally $HOME. env.get("nu.home-path") .or_else(|| env.get("nu.home-dir")) .or_else(|| env.get("HOME")) .cloned() .unwrap_or_else(|| "".to_string()) } ``` Checklist for PRs: - Are any runtime/API/UI-derived values optional? If yes, are they handled with fallbacks (not assumptions)? - Are there tests that cover absence and alternate-field spelling/runtime versions? - Does the code avoid “half-built” outputs when values are missing (e.g., empty string/inert output instead of null or malformed command text)?