Use entry model to get ID rather than variable

This fixes an issue where changing the entryType of an existing entry triggers a 500 error:

PHP Notice 'yii\base\ErrorException' with message 'Undefined index: entryId' 

in /Volumes/Sites/Personal/craft3/vendor/craftcms/cms/src/controllers/EntriesController.php:773

Stack trace:
#0 … vendor/yiisoft/yii2/base/InlineAction.php(57): craft\controllers\EntriesController->actionSwitchEntryType()
#1 … vendor/yiisoft/yii2/base/InlineAction.php(57): ::call_user_func_array:{/Volumes/Sites/Personal/craft3/vendor/yiisoft/yii2/base/InlineAction.php:57}('???', '???')
#2 … vendor/yiisoft/yii2/base/Controller.php(156): yii\base\InlineAction->runWithParams('???')
#3 … vendor/yiisoft/yii2/base/Module.php(523): yii\base\Controller->runAction('???', '???')
#4 … vendor/craftcms/cms/src/web/Application.php(246): yii\base\Module->runAction('???', '???')
#5 … vendor/craftcms/cms/src/web/Application.php(385): craft\web\Application->runAction('???', '???')
#6 … vendor/craftcms/cms/src/web/Application.php(206): craft\web\Application->_processActionRequest('???')
#7 … vendor/yiisoft/yii2/base/Application.php(380): craft\web\Application->handleRequest('???')
#8 … web/index.php(33): yii\base\Application->run()
#9 {main}

src/controllers/EntriesController.php

@@ -770,7 +770,7 @@ private function _prepEditEntryVariables(array &$variables)
         }
 
         if ($variables['entry']->id) {
-            $versions = Craft::$app->getEntryRevisions()->getVersionsByEntryId($variables['entryId'], $site->id, 1, true);
+            $versions = Craft::$app->getEntryRevisions()->getVersionsByEntryId($variables['entry']->id, $site->id, 1, true);
             /** @var EntryVersion $currentVersion */
             $currentVersion = reset($versions);