throw_of_invalid_type

The type '{0}' of the thrown expression must be assignable to 'Object'.

The analyzer produces this diagnostic when the type of the expression in a throw expression isn't assignable to Object. It isn't valid to throw null, so it isn't valid to use an expression that might evaluate to null.

The following code produces this diagnostic because s might be null:

void f(String? s) {
  throw s;
}

Add an explicit null-check to the expression:

void f(String? s) {
  throw s!;
}