fix: type 'Null' is not a subtype of type 'Object'

**Description**

`mason make` crashes after the `pre_gen` hook, outputting "type 'Null' is not a subtype of type 'Object'".
I am unable to debug or try to pinpoint this any better. The message says nothing on its own, and I am unable to understand what's going on.

So far I'm pretty sure my hooks aren't involved (pre-gen finishes correctly, post-gen isn't invoked since the crash happens before that).

I've tried my best to pinpoint the problem but I unluckily can't.

**Steps To Reproduce**
I've got a full reproducible example.

1. Go to [this repo](https://github.com/lucavenir/riverpod_clean_feature_brick)
2. Clone it
3. `mason make riverpod_clean_feature_brick`
4. See error

**Expected Behavior**
At least some explanation on why this crashed.

**Additional Context**
I'm trying to make a riverpod - related feature brick, following an architecture that we've recently adopted