check that page exists before using it (#2366)

Author: gumaerc

frontends/main/src/app-pages/DashboardPage/CoursewareDisplay/transform.test.tsx

@@ -33,7 +33,7 @@ describe("Transforming mitxonline enrollment data to DashboardResource", () => {
         coursewareId: apiData.run.courseware_id ?? null,
         type: DashboardResourceType.Course,
         title: apiData.run.title,
-        marketingUrl: apiData.run.course.page.page_url,
+        marketingUrl: apiData.run.course.page?.page_url,
         run: {
           startDate: apiData.run.start_date,
           endDate: apiData.run.end_date,

frontends/main/src/app-pages/DashboardPage/CoursewareDisplay/transform.ts

@@ -30,7 +30,7 @@ const mitxonlineEnrollment = (raw: CourseRunEnrollment): DashboardCourse => {
     coursewareId: raw.run.courseware_id ?? null,
     type: DashboardResourceType.Course,
     title: course.title,
-    marketingUrl: course.page.page_url,
+    marketingUrl: course.page?.page_url,
     run: {
       startDate: raw.run.start_date,
       endDate: raw.run.end_date,
@@ -61,7 +61,7 @@ const mitxonlineUnenrolledCourse = (
     coursewareId: run?.courseware_id ?? null,
     type: DashboardResourceType.Course,
     title: course.title,
-    marketingUrl: course.page.page_url,
+    marketingUrl: course.page?.page_url,
     run: {
       startDate: run?.start_date,
       endDate: run?.end_date,