[macros] better error message when accessing not-yet-available `ClassField.expr()`

Currently if we don't respect the build order in macros and try to match on `ClassField.expr()` return value for a field that is not yet built, we'll get a rather cryptic `Uncaught exception field access on null`, since the `expr()` method would return `null`.

Could we instead throw a "field not ready" error on `expr()` invokation?