TypeError: eff.cancel is not a function (when assigning function effect twice)

[hex13]

steps to reproduce:

        const store = createStore({
            a: init(0).on('foo', () => () => 1)
        }, createFeedbacks());
        store.dispatch({type: 'foo'});
        store.dispatch({type: 'foo'});

 TypeError: eff.cancel is not a function
  at visitNode (src/resmix.js:350:37)
  at visitNode (src/resmix.js:367:29)
  at Object.action [as dispatch] (src/resmix.js:371:17)

How it should work and why this bug occurs:

Feedbacks is designed to auto-cancel previous effect on same property.
e.g. () => new Observable(...) cancel previous observable bound to property.

But effects can be various things (including functions) and when a function is returned, Feedbacks somewhat wants to cancel previous function by calling result.cancel() when result is a result of running effect in EffectRunner. But EffectRunner doesn't always return cancellable result.

So either it should cancel previous function or just checking if eff.cancel exists in result.

[hex13]

I added checking if eff.cancel exists for now.