unnecessary_null_aware_operator_on_extension_on_nullable       
            
            Unnecessary null aware operator on extension on a nullable type.
This rule is available as of Dart 2.18.0.
Details
Avoid null aware operators for members defined in an extension on a nullable type.
BAD:
extension E on int? {
  int m() => 1;
}
f(int? i) => i?.m();
GOOD:
extension E on int? {
  int m() => 1;
}
f(int? i) => i.m();
Usage
To enable the unnecessary_null_aware_operator_on_extension_on_nullable rule,
add unnecessary_null_aware_operator_on_extension_on_nullable under linter > rules in your
analysis_options.yaml
file:
linter:
  rules:
    - unnecessary_null_aware_operator_on_extension_on_nullable