Fix undefined errors for focusing unmounted elements

[milesj]

@vadimdemedes

Ran into an issue where focusNext was called in a side-effect, but the focusables (children) were unmounted before it fired, resulting in this error. Just adding some undefined checks

error The above error occurred in the <InternalApp> component:
    in InternalApp

Consider adding an error boundary to your tree to customize error handling behavior.
Visit https://fb.me/react-error-boundaries to learn more about error boundaries.
/Users/milesj/Projects/packemon/node_modules/yoga-layout-prebuilt/yoga-layout/build/Release/nbind.js:53
        throw ex;
        ^

TypeError: Cannot read property 'id' of undefined
    at App.<anonymous> (/Users/milesj/Projects/packemon/node_modules/ink/build/components/App.js:119:70)
    at getStateFromUpdate (/Users/milesj/Projects/packemon/node_modules/react-reconciler/cjs/react-reconciler.development.js:3420:35)
    at processUpdateQueue (/Users/milesj/Projects/packemon/node_modules/react-reconciler/cjs/react-reconciler.development.js:3494:21)
    at updateClassInstance (/Users/milesj/Projects/packemon/node_modules/react-reconciler/cjs/react-reconciler.development.js:4297:5)
    at updateClassComponent (/Users/milesj/Projects/packemon/node_modules/react-reconciler/cjs/react-reconciler.development.js:8196:20)
    at beginWork$1 (/Users/milesj/Projects/packemon/node_modules/react-reconciler/cjs/react-reconciler.development.js:9962:16)
    at Object.invokeGuardedCallbackImpl (/Users/milesj/Projects/packemon/node_modules/react-reconciler/cjs/react-reconciler.development.js:11563:10)
    at invokeGuardedCallback (/Users/milesj/Projects/packemon/node_modules/react-reconciler/cjs/react-reconciler.development.js:11740:31)
    at beginWork$$1 (/Users/milesj/Projects/packemon/node_modules/react-reconciler/cjs/react-reconciler.development.js:15778:7)
    at performUnitOfWork (/Users/milesj/Projects/packemon/node_modules/react-reconciler/cjs/react-reconciler.development.js:14696:12)

[vadimdemedes]

Good catch, thanks!