As of v22.0.0 of platform-client-v2, model properties of type Map are no longer initialized by default. This will cause runtime exceptions if you use model.getMapProperty().put(key, value) without first initializing via model.mapProperty(hashMap) or model.setMapProperty(hashMap).
The reason for this change is that initialization of Map properties was causing PUT requests to overwrite existing data. The SDK was sending an empty object in the payload, which is interpreted as an intent to remove all values (as opposed to not sending the property at all, a null value).