parse-server: Cannot read property 'facebook' of null

Issue Description

Users report that they have trouble logging in and we see these entries in the NodeJS logs quite frequently since some hours:

error: Uncaught internal server error.Cannot read property 'facebook' of null
{
    "stack": "TypeError: Cannot read property 'facebook' of null\n    at Object.keys.forEach.provider (/var/app/current/node_modules/parse-server/lib/RestWrite.js:402:49)\n    at Array.forEach (<anonymous>)\n    at findUsersWithAuthData.then (/var/app/current/node_modules/parse-server/lib/RestWrite.js:400:29)\n    at process._tickCallback (internal/process/next_tick.js:68:7)"
}

We also see a few: (node:3200) MaxListenersExceededWarning: Possible EventEmitter memory leak detected. 11 connect listeners added. Use emitter.setMaxListeners() to increase limit

Maybe related to https://github.com/parse-community/parse-server/issues/2189.

Steps to reproduce

Haven’t been able to reproduce yet.

Expected Results

User should be able to log in.

Actual Outcome

Users cannot log in.

Environment Setup

• Server

  • parse-server version (Be specific! Don’t say ‘latest’.) : 3.9.0
  • Operating System: AWS Linux
  • Hardware: AWS
  • Localhost or remote server? (AWS, Heroku, Azure, Digital Ocean, etc): AWS

• Database

  • MongoDB version: 3.6.12
  • Storage engine: WiredTiger
  • Hardware: mLab
  • Localhost or remote server? (AWS, mLab, ObjectRocket, Digital Ocean, etc): mLab

Logs/Trace