Keyboard navigation is broken

[gkappen]

Keyboard navigation (using ArrowLeft and ArrowRight to go to the previous or next topic) is broken on any site created using this addon. You can check it out by going to any topic on this addons own docs and pressing one of the arrow keys.

The resulting error is something like this:

Uncaught Error:  More context objects were passed than there are dynamic segments for the route: docs.usage
    at I.applyToHandlers (vendor-907f1d49045ce1b9b3f2f6d948b40f39.js:46691:15)
    at I.applyToState (vendor-907f1d49045ce1b9b3f2f6d948b40f39.js:46628:19)
    at s.applyIntent (vendor-907f1d49045ce1b9b3f2f6d948b40f39.js:47510:21)
    at R (vendor-907f1d49045ce1b9b3f2f6d948b40f39.js:28701:45)
    at r._prepareQueryParams (vendor-907f1d49045ce1b9b3f2f6d948b40f39.js:28260:19)
    at r._doTransition (vendor-907f1d49045ce1b9b3f2f6d948b40f39.js:28219:12)
    at h.transitionTo (vendor-907f1d49045ce1b9b3f2f6d948b40f39.js:27027:37)
    at i.nextPage (vendor-907f1d49045ce1b9b3f2f6d948b40f39.js:81107:23)
    at chunk.21.0381709c1e24f2a2857d.js:2:117294
    at Array.forEach (<anonymous>)

The offending code seems to be at https://github.com/ember-learn/ember-cli-addon-docs/blob/daeaf9f909328ba92b106031b992bb085b96c23d/addon/components/docs-viewer/index.js#L55..L56 and https://github.com/ember-learn/ember-cli-addon-docs/blob/daeaf9f909328ba92b106031b992bb085b96c23d/addon/components/docs-viewer/index.js#L66..L67.

The fix seems fairly easy; If the model on a route is undefined the transitionTo method should not be called with the model argument.

Changing the code to the following fixes it, but I don't know if this might have side effects

const { route, model } = this.docsRoutes.previous;
if (model) {
  this.router.transitionTo(route, model);
} else {
  this.router.transitionTo(route);
}

[RobbieTheWagner]

@gkappen sorry for the huge delay here! This fix seems reasonable to me. Would you like to open a PR?