Better error message when missing entity mapping

Otherwise you just get an error like

> php.CRITICAL: Call to a member function getClassMetadata() on null

Repository/ServiceEntityRepository.php

@@ -4,6 +4,7 @@
 
 use Doctrine\Common\Persistence\ManagerRegistry;
 use Doctrine\ORM\EntityRepository;
+use \LogicException;
 
 /**
  * Optional EntityRepository base class with a simplified constructor (for autowiring).
@@ -28,6 +29,13 @@ public function __construct(ManagerRegistry $registry, $entityClass)
     {
         $manager = $registry->getManagerForClass($entityClass);
 
+        if (null === $manager) {
+            throw new LogicException(sprintf(
+                'Could not find the entity manager for class "%s". Check your Doctrine configuration to make sure it is configured to load this entity’s metadata.',
+                $entityClass
+            ));
+        }
+
         parent::__construct($manager, $manager->getClassMetadata($entityClass));
     }
 }

Tests/Repository/ServiceEntityRepositoryTest.php

@@ -0,0 +1,20 @@
+<?php
+
+namespace Doctrine\Bundle\DoctrineBundle\Tests\Repository;
+
+use Doctrine\Bundle\DoctrineBundle\Repository\ServiceEntityRepository;
+use Doctrine\Common\Persistence\ManagerRegistry
+use PHPUnit\Framework\TestCase;
+
+class ServiceEntityRepositoryTest extends TestCase
+{
+    /**
+     * @expectedException \LogicException
+     * @expectedExceptionMessage Could not find the entity manager for class "Doctrine\Bundle\DoctrineBundle\Tests\Repository\TestEntity". Check your Doctrine configuration to make sure it is configured to load this entity’s metadata.
+     */
+    public function testConstructorThrowsExceptionWhenNoManagerFound()
+    {
+        $registry = $this->getMockBuilder(ManagerRegistry::class)->getMock();
+        new ServiceEntityRepository($registry, TestEntity::class);
+    }
+}