MAGETWO-62032: Empty default website error

Author: vzabaznov

app/code/Magento/Backend/Block/System/Store/Edit/Form/Website.php

@@ -108,7 +108,7 @@ protected function _prepareStoreFieldset(\Magento\Framework\Data\Form $form)
             );
         }
 
-        if (!$websiteModel->getIsDefault() && $websiteModel->getStoresCount()) {
+        if ($this->checkIsSingleAndIsDefaultStore($websiteModel)) {
             $fieldset->addField(
                 'is_default',
                 'checkbox',
@@ -133,4 +133,12 @@ protected function _prepareStoreFieldset(\Magento\Framework\Data\Form $form)
             ['name' => 'website[website_id]', 'value' => $websiteModel->getId()]
         );
     }
+
+    private function checkIsSingleAndIsDefaultStore($websiteModel)
+    {
+        $hasOnlyDefaultStore = $websiteModel->getStoresCount() == 1 &&
+            isset($websiteModel->getStoreIds()[\Magento\Store\Model\Store::DEFAULT_STORE_ID]);
+
+        return !$websiteModel->getIsDefault() && $websiteModel->getStoresCount() && !$hasOnlyDefaultStore;
+    }
 }