DocumentArray#create() throws an error when creating a subdoc with a manually populated path

[vkarpov15]

Do you want to request a feature or report a bug?

Bug

What is the current behavior?

When checking SchemaType#_isRef(), fullPath() may be undefined if the subdoc was just create() -ed. This means the below script:

'use strict';
  
const mongoose = require('mongoose');
const { Schema } = mongoose;

run().catch(err => console.log(err));

async function run() {
  console.log(mongoose.version);
  await mongoose.connect('mongodb://localhost:27017/test');
  await mongoose.connection.dropDatabase();

  const schema = new Schema({ subdoc: { ref: 'Test2', type: 'ObjectId' }, arr: [{ num: Number }] });
  const Test = mongoose.model('Test', schema);
  const Test2 = mongoose.model('Test2', Schema({ name: String }));

  const doc2 = await Test2.create({ name: 'test' });
  const doc = await Test.create({ subdoc: doc2, arr: [] });

  const subdoc = doc.arr.create({ num: '42' });

  await subdoc.validate();

  doc.arr.push(subdoc);
  await doc.save();
  console.log('Done');
}

Throw an error on the doc.arr.create() call:

TypeError: Cannot read property 'endsWith' of null
    at model.Document.populated (/home/val/Workspace/MongoDB/mongoose/lib/document.js:4165:24)
    at Function.SchemaType._isRef (/home/val/Workspace/MongoDB/mongoose/lib/schematype.js:1388:17)
    at ObjectId.cast (/home/val/Workspace/MongoDB/mongoose/lib/schema/objectid.js:225:18)
    at ObjectId.SchemaType.applySetters (/home/val/Workspace/MongoDB/mongoose/lib/schematype.js:1122:12)
    at ObjectId.SchemaType.getDefault (/home/val/Workspace/MongoDB/mongoose/lib/schematype.js:1069:25)
    at $__applyDefaults (/home/val/Workspace/MongoDB/mongoose/lib/document.js:509:24)
    at EmbeddedDocument.Document (/home/val/Workspace/MongoDB/mongoose/lib/document.js:138:7)
    at EmbeddedDocument.Subdocument (/home/val/Workspace/MongoDB/mongoose/lib/types/subdocument.js:27:12)
    at EmbeddedDocument.ArraySubdocument [as constructor] (/home/val/Workspace/MongoDB/mongoose/lib/types/ArraySubdocument.js:36:15)
    at new EmbeddedDocument (/home/val/Workspace/MongoDB/mongoose/lib/schema/documentarray.js:115:17)

If the current behavior is a bug, please provide the steps to reproduce.

What is the expected behavior?

No error

What are the versions of Node.js, Mongoose and MongoDB you are using? Note that "latest" is not a version.

Node.js 14.4
Mongoose 6.0.6
MongoDB 4.4.0