fix: correct return type of useChain hook

[2wheeh]

With current tsconfig ("strictNullChecks": false), TS infers return type of Array.find() as non-nullable, even though it can return undefined at runtime.

This leads type mismatch here:

const walletStore = this.wallets.find(w => w.info.name === walletName);

This returns undefined as well.

There are more potential type mismatches of this sort.
Turning on strictNullChecks would be a more desirable solution in the long term.