Null check operator used on a null value error flutter

Question

i am trying to implement the registration activity in the app and working with the textFields. My validator is throwing "Null check operator used on a null value" in the line: String error = widget.validateFunction(val)!;

if I remove the '!' it gives me error: A value of type 'String?' can't be assigned to a variable of type 'String'. How do I work around it? Thank you!