Allow loading with no unit, and of sequences with no unitIds. (#34)

This requires some defensive programming here and there to let things load in a minimal state.

src/data/api.js

@@ -41,14 +41,14 @@ function normalizeBlocks(courseUsageKey, blocks) {
         models.courses[block.id] = {
           id: courseUsageKey,
           title: block.display_name,
-          sectionIds: block.children,
+          sectionIds: block.children || [],
         };
         break;
       case 'chapter':
         models.sections[block.id] = {
           id: block.id,
           title: block.display_name,
-          sequenceIds: block.children,
+          sequenceIds: block.children || [],
         };
         break;
 
@@ -57,7 +57,7 @@ function normalizeBlocks(courseUsageKey, blocks) {
           id: block.id,
           title: block.display_name,
           lmsWebUrl: block.lms_web_url,
-          unitIds: block.children,
+          unitIds: block.children || [],
         };
         break;
       case 'vertical':

src/data/thunks.js

@@ -5,7 +5,7 @@ import {
   getSequenceMetadata,
 } from './api';
 import {
-  addModelsMap, updateModel, updateModels,
+  addModelsMap, updateModel, updateModels, updateModelsMap,
 } from '../model-store';
 import {
   fetchCourseRequest,
@@ -40,11 +40,12 @@ export function fetchCourse(courseUsageKey) {
         modelType: 'sections',
         modelsMap: sections,
       }));
-      dispatch(addModelsMap({
+      // We update for sequences and units because the sequence metadata may have come back first.
+      dispatch(updateModelsMap({
         modelType: 'sequences',
         modelsMap: sequences,
       }));
-      dispatch(addModelsMap({
+      dispatch(updateModelsMap({
         modelType: 'units',
         modelsMap: units,
       }));