[Cashtab] Attempt to patch whitescreen failures related to OrderBook.js and hex

Summary:
Getting repeated reports of this failure but have not been able to repeat. Based on what I am seeing in the dev log, failure seems related to hex.ts as it is used in OrderBook.js --- which means a hex method from ecash-lib.

Hex methods in this screen only used in one place on load. Validate input to make sure it is loaded before calculating isMaker.

Test Plan:
npm test to confirm behavior is unchanged

this should only catch an edge case that happens sometimes while app is loading

Reviewers: #bitcoin_abc, johnkuney

Reviewed By: #bitcoin_abc, johnkuney

Differential Revision: https://reviews.bitcoinabc.org/D17003