Flutter compile error: non-null value must be returned since the return type 'String' doesn't allow null - displayString

Question

My tests are failing to compile and run in a Dart only project that is referenced by my Flutter project. I'm receiving the following error message

Failed to precompile test:test:
../../../../../../../../../.pub-cache/hosted/pub.dartlang.org/analyzer-1.0.0/lib/src/error/best_practices_verifier.dart:1952:14: Error: A non-null value must be returned since the return type 'String' doesn't allow null.
  String get displayString {

Any ideas please?

Flutter 2.2.1 (current stable channel)
Tools • Dart 2.13.1

(I've asked the question in Flutter's github here also) https://github.com/flutter/flutter/issues/83683

Answer 1

There was a component using analyzer version 1.0.0

Upgrading this component to use analyzer version 1.7 or above seemed to fix the problem for me.