No useful error when attempting to preview course with empty children

[tomgreenfield]

On develop branch, attempt to preview a course with no content objects, or a block with no components etc.

Erroring function:

adapt_authoring/lib/outputmanager.js

Lines 199 to 272 in b86cac9

	OutputPlugin.prototype.sanitizeCourseJSON = function(mode, json, next) {
	var self = this;
	try {
	var outputJson = json;
	var courseJson = outputJson['course'][0];
	var configJson = outputJson['config'][0];
	var contentObjectJson = outputJson['contentobject'];
	var blockJson = outputJson['block'];
	var componentJson = outputJson['component'];
	var courseId = courseJson._id;
	// The Adapt Framework expects the 'type' and '_id'
	// attributes of the course to be set to 'course'
	courseJson._type = 'course';
	courseJson._id = 'course';
	courseJson._latestTrackingId = blockJson.length;
	// Replace any reference to the original course _id value in contentObjects JSON
	for (var i = 0; i < contentObjectJson.length; i++) {
	if (contentObjectJson[i]._parentId.toString() == courseId) {
	contentObjectJson[i]._parentId = 'course';
	}
	}
	// Add a _trackingId value to every block
	for (var i = 0; i < blockJson.length; i++) {
	blockJson[i]._trackingId = (i + 1);
	}
	// The 'properties' property of a component should not be included as an
	// attribute in the output, but all its children should
	for (var i = 0; i < componentJson.length; i++) {
	if (componentJson[i].hasOwnProperty('properties')) {
	for(var key in componentJson[i].properties){
	if (componentJson[i].properties.hasOwnProperty(key)){
	componentJson[i][key] = componentJson[i].properties[key];
	}
	}
	// Remove the 'properties' property
	delete componentJson[i].properties;
	}
	}
	async.waterfall([
	function(callback) {
	if (mode === Constants.Modes.export) {
	return callback();
	}
	self.generateIncludesForConfig(configJson, function(error, includes) {
	if (error) {
	return callback(error);
	}
	configJson.build = { includes: includes };
	callback();
	});
	}
	], function(error) {
	if (error) {
	logger.log(error);
	return next(error);
	}
	// Store the sanitized JSON
	Object.assign(outputJson, {
	course: courseJson,
	config: configJson,
	contentobject: contentObjectJson,
	component: componentJson
	});
	next(null, outputJson);
	});
	} catch(err) {
	return next(err);
	}
	};

Frontend output:

Backend output:

info: [06 Sep 2018 12:25:38 +01:00] Retrieving JSON
error: [06 Sep 2018 12:27:27 +01:00] TypeError: Cannot read property 'length' of undefined