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Safari throws this error when JavaScript tries to access `window.webkit.messageHandlers`, which only exists inside WKWebView when native iOS/macOS apps register a handler. The issue typically occurs when WKWebView-specific code runs in a normal browser context. Fix it by wrapping calls with feature detection using optional chaining or explicit checks before accessing the bridge. If you control the native app, verify the handler is registered correctly with the exact name before JavaScript executes. Monitor production errors to determine if this affects real users or is harmless noise from unsupported contexts.