Parameter can't have a value of null in flutter
Y ou can also use positional parameters (no curly brackets) that are mandatory by default, so not nullable. void calculate( int factor) { // ... } and is called without naming the parameter: calculate( 12 ); These kind of parameters can be used on constructors this way: class Foo extends StatelessWidget { final String myVar; const Foo( this .myVar, {Key? key}): super (key: key); // ... } and "can be followed either by named parameters OR by optional positional parameters (but not both)" , see doc here: dart parameters