How to add a World Object Reference to a function?

I have a widget with buttons to select which level to load.

As this is repetitive code I would like to use a function. This function needs a input parameter referencing the map object.

From the Open Level (By Object Reference) function I can see there does exist a object type for this (World Soft Object Reference), however I am unable to specify this object type as a parameter?

Currently I am getting around this by keeping this object type as a global variable in the widget, and then referring to it in my function, however it doesn’t feel to be best practice.