missing_default_value_for_parameter

The parameter '{0}' can't have a value of 'null' because of its type, but the implicit default value is 'null'.

With null safety, use the 'required' keyword, not the '@required' annotation.

The analyzer produces this diagnostic when an optional parameter, whether positional or named, has a potentially non-nullablePotentially non-nullableA type that is either non-nullable explicitly or due to being a type parameter. Learn more type and doesn't specify a default value. Optional parameters that have no explicit default value have an implicit default value of null. If the type of the parameter doesn't allow the parameter to have a value of null, then the implicit default value isn't valid.

The following code produces this diagnostic because x can't be null, and no non-null default value is specified:

void f([int x]) {}

As does this:

void g({int x}) {}

If you want to use null to indicate that no value was provided, then you need to make the type nullable:

void f([int? x]) {}
void g({int? x}) {}

If the parameter can't be null, then either provide a default value:

void f([int x = 1]) {}
void g({int x = 2}) {}

or make the parameter a required parameter:

void f(int x) {}
void g({required int x}) {}