there is an error ("Cannot call method 'db' of null") when npm start

[bobniq]

TypeError: Cannot call method 'db' of null
at F:\github\angular-directory\server\routes\employee.js:7:22
at F:\github\angular-directory\server\node_modules\mongodb\lib\mongodb\mongo_client.js:99:20
at F:\github\angular-directory\server\node_modules\mongodb\lib\mongodb\db.js:285:18
at F:\github\angular-directory\server\node_modules\mongodb\lib\mongodb\db.js:363:18
at Server.close (F:\github\angular-directory\server\node_modules\mongodb\lib\mongodb\connection\server.js:192:38)
at Db.close (F:\github\angular-directory\server\node_modules\mongodb\lib\mongodb\db.js:359:21)
at F:\github\angular-directory\server\node_modules\mongodb\lib\mongodb\db.js:283:21
at null. (F:\github\angular-directory\server\node_modules\mongodb\lib\mongodb\connection\server.js:559:7)
at emit (events.js:106:17)
at null. (F:\github\angular-directory\server\node_modules\mongodb\lib\mongodb\connection\connection_pool.js:150:15)