MAGETWO-58255: Unable to view signed up billing agreement on storefront

 - Added validation for empty quote

Author: YevSent

app/code/Magento/Vault/Model/Method/Vault.php

@@ -579,7 +579,7 @@ public function assignData(\Magento\Framework\DataObject $data)
     public function isAvailable(\Magento\Quote\Api\Data\CartInterface $quote = null)
     {
         return $this->getVaultProvider()->isAvailable($quote)
-            && $this->config->getValue(self::$activeKey, $this->getStore() ?: $quote->getStoreId());
+            && $this->config->getValue(self::$activeKey, $this->getStore() ?: ($quote ? $quote->getStoreId() : null));
     }
 
     /**

app/code/Magento/Vault/Test/Unit/Model/Method/VaultTest.php

@@ -274,4 +274,32 @@ public function isAvailableDataProvider()
             ['isAvailableProvider' => true, 'isActiveVault' => true, 'expected' => true],
         ];
     }
+
+    /**
+     * @covers \Magento\Vault\Model\Method\Vault::isAvailable
+     */
+    public function testIsAvailableWithoutQuote()
+    {
+        $quote = null;
+
+        $vaultProvider = $this->getMockForAbstractClass(MethodInterface::class);
+        $config = $this->getMockForAbstractClass(ConfigInterface::class);
+
+        $vaultProvider->expects(static::once())
+            ->method('isAvailable')
+            ->with($quote)
+            ->willReturn(true);
+
+        $config->expects(static::once())
+            ->method('getValue')
+            ->with('active', $quote)
+            ->willReturn(false);
+
+        /** @var Vault $model */
+        $model = $this->objectManager->getObject(Vault::class, [
+            'config' => $config,
+            'vaultProvider' => $vaultProvider
+        ]);
+        static::assertFalse($model->isAvailable($quote));
+    }
 }