refactor(neon): throw an error if neonUserAgent hasn't been set up

Signed-off-by: Nikolas Rimikis <leptopoda@users.noreply.github.com>

packages/neon/neon/lib/src/utils/user_agent.dart

@@ -1,8 +1,11 @@
 import 'package:meta/meta.dart';
 import 'package:package_info_plus/package_info_plus.dart';
 
-late String? _userAgent;
+String? _userAgent;
 
+/// Sets the user agent.
+///
+/// It can be accessed with [neonUserAgent].
 @internal
 void buildUserAgent(final PackageInfo packageInfo) {
   var buildNumber = packageInfo.buildNumber;
@@ -12,5 +15,13 @@ void buildUserAgent(final PackageInfo packageInfo) {
   _userAgent = 'Neon ${packageInfo.version}+$buildNumber';
 }
 
+/// Gets the current user agent.
+///
+/// It must be set by calling [buildUserAgent] before. If not set a [StateError] will be thrown.
 @internal
-String get neonUserAgent => _userAgent!;
+String get neonUserAgent {
+  if (_userAgent == null) {
+    throw StateError('The user agent has not been set up. Please use `buildUserAgent` before.');
+  }
+  return _userAgent!;
+}