Fix build

Author: ondrejmirtes

Diff for: src/Type/Symfony/ServiceDynamicReturnTypeExtension.php

@@ -18,6 +18,7 @@
 use Symfony\Component\DependencyInjection\ParameterBag\ParameterBag;
 use function class_exists;
 use function in_array;
+use function is_string;
 
 final class ServiceDynamicReturnTypeExtension implements DynamicMethodReturnTypeExtension
 {
@@ -144,7 +145,17 @@ private function getHasTypeFromMethodCall(
 
 	private function determineServiceClass(ParameterBag $parameterBag, ServiceDefinition $service): ?string
 	{
-		return $parameterBag->resolveValue($service->getClass());
+		$class = $service->getClass();
+		if ($class === null) {
+			return null;
+		}
+
+		$value = $parameterBag->resolveValue($class);
+		if (!is_string($value)) {
+			return null;
+		}
+
+		return $value;
 	}
 
 }