if (token) {
    try {
        var decoded = jsonwebtoken.verify(token, jwt_key);
        req.user = decoded.user;///
         console.log(req.user._id);// i get id here and from this id i can find user in database
        UserSchema.findOne({
                _id: req.user._id
            })///this query failed finding result
            .exec(function(err, result) {
                if (err) {
                    console.log(err);//error gets output in the console.
                    res.status(500);
                    return res.json({
                        result: err
                    });
                }
                if (result) {
                    return next();
                } else {
                    res.status(400);
                    return res.json({
                        message: "The user does not exists"
                    });
                }
            });

    } catch (err) {
        res.status(403);
        res.json({
            message: "Invalid Token"
        });
    }
} else {
    res.status(401);
    res.json({
        message: "No token provided"
    });
}

{ [CastError: Cast to ObjectId failed for value "5875d76e1df97635623061c5" at path "_id" for model "User"]
  message: 'Cast to ObjectId failed for value "5875d76e1df97635623061c5" at path "_id" for model "User"',
  name: 'CastError',
  stringValue: '"5875d76e1df97635623061c5"',
  kind: 'ObjectId',
  value: '5875d76e1df97635623061c5',
  path: '_id',
  reason: undefined,
  model:
   { [Function: model]
     hooks: Kareem { _pres: {}, _posts: {} },
     base:
      Mongoose {
        connections: [Object],
        plugins: [],
        models: [Object],
        modelSchemas: [Object],
        options: [Object] },
     modelName: 'User',
     model: [Function: model],
     db:
      NativeConnection {
        base: [Object],
        collections: [Object],
        models: [Object],
        config: [Object],
        replica: false,
        hosts: null,
        host: 'localhost',
        port: 27017,
        user: undefined,
        pass: undefined,
        name: 'myschoolbus_demo',
        options: [Object],
        otherDbs: [],
        _readyState: 1,
        _closeCalled: false,
        _hasOpened: true,
        _listening: false,
        db: [Object] },
     discriminators: undefined,
     schema:
      Schema {
        obj: [Object],
        paths: [Object],
        subpaths: {},
        virtuals: [Object],
        singleNestedPaths: {},
        nested: [Object],
        inherits: {},
        callQueue: [Object],
        _indexes: [],
        methods: [Object],
        statics: {},
        tree: [Object],
        _requiredpaths: undefined,
        discriminatorMapping: undefined,
        _indexedpaths: undefined,
        query: {},
        childSchemas: [],
        s: [Object],
        options: [Object],
        '$globalPluginsApplied': true },
     collection:
      NativeCollection {
        collection: [Object],
        opts: [Object],
        name: 'users',
        collectionName: 'users',
        conn: [Object],
        queue: [],
        buffer: false,
        emitter: [Object] },
     Query: { [Function] base: [Object] },
     '$__insertMany': [Function],
     insertMany: [Function] } }