Fix GET /db/level/:handle/session

When making a course session and user has a HoC course instance (no associated classroom)

server/middleware/levels.coffee

@@ -52,7 +52,9 @@ module.exports =
       classroomMap[classroom.id] = classroom for classroom in classrooms
       levelOriginal = level.get('original')
       for courseInstance in courseInstances
-        classroom = classroomMap[courseInstance.get('classroomID').toString()]
+        classroomID = courseInstance.get('classroomID')
+        continue unless classroomID
+        classroom = classroomMap[classroomID.toString()]
         courseID = courseInstance.get('courseID')
         classroomCourse = _.find(classroom.get('courses'), (c) -> c._id.equals(courseID))
         for courseLevel in classroomCourse.levels

spec/server/functional/level.spec.coffee

@@ -80,6 +80,12 @@ describe 'GET /db/level/:handle/session', ->
       expect(res.statusCode).toBe(200)
       expect(body.codeLanguage).toBe('python')
       done()
+      
+    it 'does not break if the user has a courseInstance without an associated classroom', utils.wrap (done) ->
+      yield @courseInstance.update({$unset: {classroomID: ''}})
+      [res, body] = yield request.getAsync { uri: @url, json: true }
+      expect(res.statusCode).toBe(402)
+      done()
 
     it 'returns 402 if the user is not in a course with that level', utils.wrap (done) ->
       otherStudent = yield utils.initUser({role: 'student'})