Patch-Magento_Eav-M2.3-api-product-multiselect.patch

--- Model/TypeLocator/SimpleType.orig.php	2019-03-15 05:07:42.000000000 +0300
+++ Model/TypeLocator/SimpleType.php	2019-05-31 11:54:59.051146986 +0300
@@ -50,6 +50,11 @@
         } catch (NoSuchEntityException $exception) {
             return TypeProcessor::NORMALIZED_ANY_TYPE;
         }
+
+        if ($attribute->getFrontendInput() === 'multiselect') {
+            return TypeProcessor::NORMALIZED_ANY_TYPE;
+        }
+
         $backendType = $attribute->getBackendType();
         $backendTypeMap = [
             'static' => TypeProcessor::NORMALIZED_ANY_TYPE,