Cannot read navigateToAmpUrl of null

[sparhami]

this.navigation_ is always null when read here:

amphtml/extensions/amp-next-page/0.1/next-page-service.js

Lines 317 to 318 in a348bbc

	const a2a =
	this.navigation_.navigateToAmpUrl(next.ampUrl, 'content-discovery');

The private field is declared here:

amphtml/extensions/amp-next-page/0.1/next-page-service.js

Lines 91 to 92 in a348bbc

	/** @private {?../../../src/service/navigation.Navigation} */
	this.navigation_ = null;

But the field is never initialized in the file. This error can be reproduced clicking on a link within an <amp-next-page>.

Since this error happens in a click listener, it will cause other click listeners for the event to not be invoked.

This was broken in #17600