invalid_use_of_null_value

An expression whose value is always 'null' can't be dereferenced.

The analyzer produces this diagnostic when an expression whose value will always be null is dereferenced.

The following code produces this diagnostic because x will always be null:

int f(Null x) {
  return x.length;
}

If the value is allowed to be something other than null, then change the type of the expression:

int f(String? x) {
  return x!.length;
}