If a classic notebook contains a `Mutable` ```js mutable foo = null ``` then importing the cell into a notebook via ```js import {foo} from "observable:…" ``` and displaying it via ```js foo ``` fails with the error ``` RuntimeError: null is not an object (evaluating '_.generator') ``` ### Minimal repro ```html <!doctype html> <notebook> <script type="module"> import {foo} from "observable:d/3a120539ecaa0449@70" </script> <script type="module"> foo </script> </notebook> ```