fix: add presence check for childResourceType

[MrSunshyne]

Solves #4

This PR resolves an issue where sometimes navigating within the GUI causes a blocking error with .fields being undefined. I'm not sure if this is fixing the issue itself or a side effect of the issue.

Note that Typescript is also complaining at this exact spot if you run it without the fix.

How to run locally ?

1. Clone moquerie on your machine for example at /home/user/Projects/github.com/project-name/moquerie/
2. Build moquerie : pnpm run build
3. In the project where moquerie is being used update the scripts section in packages.json as follows to point towards your fork

scripts: {
    "dev:mock": "node /home/user/Projects/github.com/project-name/moquerie/packages/moquerie/bin.mjs",
    ...
 }

[phsultan]

This indeed fixes it, thanks @MrSunshyne !