getting error TypeError: Cannot convert undefined or null to object when starting application

[joshStillerman]

In my model below the properties I have:

"mixins": {
      "ReadOnly" : {
        "entered" : true,
        "dbkey" : true
      }
    }

and when I run the application I get:

$ node .
/Users/jas/loopback/hts/node_modules/loopback-ds-readonly-mixin/lib/read-only.js:96
    Object.keys(Model.definition.settings.relations).forEach(relationName => {
           ^

TypeError: Cannot convert undefined or null to object
    at Function.keys (<anonymous>)
    at EventEmitter.Model.on (/Users/jas/loopback/hts/node_modules/loopback-ds-readonly-mixin/lib/read-only.js:96:12)
    at emitOne (events.js:96:13)
    at EventEmitter.emit (events.js:188:7)
    at Function.app.model (/Users/jas/loopback/hts/node_modules/loopback/lib/application.js:162:9)
    at /Users/jas/loopback/hts/node_modules/loopback-boot/lib/executor.js:203:9
    at Array.forEach (native)
    at setupModels (/Users/jas/loopback/hts/node_modules/loopback-boot/lib/executor.js:199:23)
    at execute (/Users/jas/loopback/hts/node_modules/loopback-boot/lib/executor.js:40:3)
    at bootLoopBackApp (/Users/jas/loopback/hts/node_modules/loopback-boot/index.js:154:3)
    at Object.<anonymous> (/Users/jas/loopback/hts/server/server.js:23:1)
    at Module._compile (module.js:571:32)
    at Object.Module._extensions..js (module.js:580:10)
    at Module.load (module.js:488:32)
    at tryModuleLoad (module.js:447:12)
    at Function.Module._load (module.js:439:3)

Is this a bug, or am I just missing something ?
I am using:

    "loopback": "^3.19.0",
    "loopback-boot": "^2.6.5",
    "loopback-component-explorer": "^6.0.0",
    "loopback-connector-mssql": "^2.9.0",
    "loopback-ds-readonly-mixin": "^2.0.4",

I commented out the part about updates via relationship, and it now does not choke. However, when I try to insert a record using a PUT, and leave out readonly fields (because they are provided by the database) I get:

mdsdev4:hts jas$ node .
Web server listening at: http://localhost:3000
Browse your REST API at http://localhost:3000/explorer
Unhandled error for request POST /api/Runs?access_token=heyhey: ValidationError: The `Runs` instance is not valid. Details: `username` can't be blank (value: undefined); `entered` can't be blank (value: undefined); `dbkey` can't be blank (value: undefined).