BUGFIX: node:repair fails with could not be converted to string

Fixes the following crash during node:repair

```shell
./flow node:repair --dry-run --only removeBrokenEntityReferences
Dry run, not committing any changes.

Checking for broken entity references ...
Object of class Neos\Flow\Persistence\Doctrine\Proxies\__CG__\Neos\Media\Domain\Model\ImageVariant could not be converted to string

  Type: Error
  File: Data/Temporary/Development/SubContextWbWeb/Cache/Code/Flow_Object_Classes/Neos_ContentRepository_Command_NodeCommandControllerPlugin.php
  Line: 836
```

resolved #4794

Neos.ContentRepository/Classes/Command/NodeCommandControllerPlugin.php

@@ -833,7 +833,16 @@ public function removeBrokenEntityReferences($workspaceName)
                             $convertedProperty->__load();
                         } /** @noinspection PhpRedundantCatchClauseInspection */ catch (EntityNotFoundException $e) {
                             $nodesWithBrokenEntityReferences[$nodeData->getIdentifier()][$propertyName] = $nodeData;
-                            $this->dispatch(self::EVENT_NOTICE, sprintf('Broken reference in "<i>%s</i>" (%s), property "<i>%s</i>" (<i>%s</i>) referring to <i>%s</i>.', $nodeData->getPath(), $nodeData->getIdentifier(), $propertyName, $propertyType, $propertyValue));
+                            $this->dispatch(self::EVENT_NOTICE, sprintf(
+                                'Broken reference in "<i>%s</i>" (%s), property "<i>%s</i>" (<i>%s</i>)%s.',
+                                $nodeData->getPath(),
+                                $nodeData->getIdentifier(),
+                                $propertyName,
+                                $propertyType,
+                                method_exists($propertyValue, '__toString') ?
+                                    ' referring to <i>' . $propertyValue->__toString() . '</i>' :
+                                    ''
+                            ));
                             $brokenReferencesCount ++;
                         }
                     }