Populate with retainNullValues enabled & omitting _id throws Cannot read property '$__setValue' of null

[FelixRe0]

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

What is the current behavior?
Using populate + omitting _id results in the following error if the populated document does not exist and retainNullValues is set to true:

TypeError: Cannot read property '$__setValue' of null
   at maybeRemoveId (\node_modules\mongoose\lib\helpers\populate\assignVals.js:237:23)
   at valueFilter (\node_modules\mongoose\lib\helpers\populate\assignVals.js:193:7)
   at setValue (\node_modules\mongoose\lib\helpers\populate\assignVals.js:67:12)

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

 const options = {
     model: 'Model',
     select: 'title -_id',
     options: { retainNullValues: true },
   }
  let user = await User.findById(_id , 'arrayOfObjectIds').populate({ options, path: 'arrayOfObjectIds' })

What is the expected behavior?

Returning array with populated document/null

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

Node v12.13.1
Mongoose ^5.10.0
MongoDB v4.2.8

I suspect that subdoc should be checked for value null in maybeRemoveId in assignVals.js