Cannot read property 'error' of null in realtime-subsription-handshake-link.js

[simonbuerger]

Seems to happen after leaving app open for an extended period (eg in simulator, but I have seen it on device too) and observer is set to null when it tries to call observer.error

Think it might be this line, but I noticed there's no null check on any of the observer.error calls

aws-mobile-appsync-sdk-js/packages/aws-appsync-subscription-link/src/realtime-subscription-handshake-link.ts

Line 722 in f5c6e1a

observer.error({

Can I open a PR and add null checks for the observer.error calls?