react-redux-v6: withRef is removed. To access the wrapped instance, use a ref on the connected component

You need to replace withRef with forwardRef as per the release notes:

The withRef option to connect has been replaced with forwardRef. If {forwardRef : true} has been passed to connect, adding a ref to the connected wrapper component will actually return the instance of the wrapped component.

So in your case:

export default connect(
  mapStateToProps, mapDispatchToProps, null, {forwardRef: true}
)(InviteReceiverForm)