fix: unit object not always ready when course loaded

If we don’t yet have the course blocks API loaded when the course metadata finishes, `unit` will be undefined still, and `unit.id` will fail.

src/courseware/course/Course.jsx

@@ -32,7 +32,6 @@ function Course({
   const course = useModel('courses', courseId);
   const sequence = useModel('sequences', sequenceId);
   const section = useModel('sections', sequence ? sequence.sectionId : null);
-  const unit = useModel('units', unitId);
 
   useLogistrationAlert();
   useEnrollmentAlert(courseId);
@@ -60,7 +59,7 @@ function Course({
         />
         {isStaff && (
         <InstructorToolbar
-          unitId={unit.id}
+          unitId={unitId}
         />
         )}
         <CourseTabsNavigation tabs={tabs} activeTabSlug="courseware" />

src/courseware/course/InstructorToolbar.jsx

@@ -53,7 +53,7 @@ const mapStateToProps = (state, props) => {
 
   const activeUnit = state.models.units[props.unitId];
   return {
-    activeUnitLmsWebUrl: activeUnit.lmsWebUrl,
+    activeUnitLmsWebUrl: activeUnit ? activeUnit.lmsWebUrl : undefined,
   };
 };