ENGCOM-3609: Fixed fatal error if upgrading from Magento v2.0.0 to v2.3 and non system attributes missing #19530

 - Merge Pull Request #19530 from suneet64/magento2:configurable-product-attribute-fix
 - Merged commits:
   1. eb05687

Author: magento-engcom-team

app/code/Magento/ConfigurableProduct/Setup/Patch/Data/InstallInitialConfigurableAttributes.php

@@ -64,18 +64,21 @@ public function apply()
             'color'
         ];
         foreach ($attributes as $attributeCode) {
-            $relatedProductTypes = explode(
-                ',',
-                $eavSetup->getAttribute(\Magento\Catalog\Model\Product::ENTITY, $attributeCode, 'apply_to')
-            );
-            if (!in_array(Configurable::TYPE_CODE, $relatedProductTypes)) {
-                $relatedProductTypes[] = Configurable::TYPE_CODE;
-                $eavSetup->updateAttribute(
-                    \Magento\Catalog\Model\Product::ENTITY,
-                    $attributeCode,
-                    'apply_to',
-                    implode(',', $relatedProductTypes)
+            $attribute = $eavSetup->getAttribute(\Magento\Catalog\Model\Product::ENTITY, $attributeCode, 'apply_to');
+            if ($attribute) {
+                $relatedProductTypes = explode(
+                    ',',
+                    $attribute
                 );
+                if (!in_array(Configurable::TYPE_CODE, $relatedProductTypes)) {
+                    $relatedProductTypes[] = Configurable::TYPE_CODE;
+                    $eavSetup->updateAttribute(
+                        \Magento\Catalog\Model\Product::ENTITY,
+                        $attributeCode,
+                        'apply_to',
+                        implode(',', $relatedProductTypes)
+                    );
+                }
             }
         }
     }