The function can't be unconditionally invoked because it can be 'null'. The '!' will have no effect because the receiver can't be null

There are few ways I suggest. You can use any one of them

1.You can change it like

if(widget.toggleView != null){
    widget.toggleView();
} 

2.add require in constructor to make toggleView cannot be null

Register({require this.toggleView});

3.if you don't want to change code and guarantee toggleView will never be null. Try this

onPressed: widget.toggleView!

One more thing change is

///final Function? toggleView; should not be accepted for onPressed
///you can use VoidCallback or Function()
final VoidCallback? toggleView;
/// final Function()? toggleView;

Use VoidCallback or Function() instead of Function for the onPressed, and it should work