Unable to define type objects whichs default may be null

[Redge1973]

I can't manage to specify the properties for an object that can be null. I am building the config and the json schema from a json and I really don't know what to specify as type. If I enter the type mixed, then the correct error message is displayed for the value null, but a date schema is generated for it, which of course is not accepted if the value is an object with e.g. {index: 1, value: 2}.
validation: {
errorMessages: {
required: 'Please enter an interval',
},
required: true,
properties: {
type: 'mixed',
},
},
defaultValue: null,
I just can't find any help in the documentation on how to do this. If I take object as type, I always get the message null is not allowed. isNullable: true doesn't help either