MutableRef

For use with useRef() hook which is slightly different than the mutable Ref. current is designed to be set directly or can be used on the ref attribute for a component. Since you can provide an initial value, if you want null to be valid, either use null directly or model it explicitly as T|Null to force client's to deal with the explict null value (otherwise they need to check for the null value directly).

Reconcile with ReactRef perhaps rename the member to something like unsafe.