Param

I am trying to get a param. But I get this error:The method 'where' was called on null. Receiver: null. Tried calling: where(Closure: (dynamic) => bool). This how I am trying to get the param:
Param _param = Param(ros:ros,
      name: 'tank_max'
    );
var tmp=_param.get();

I appreciate any help.
