Correct the error message in case $block is still created but is not an AbstractBlock

Author: Hiep Ho Minh

lib/internal/Magento/Framework/View/Layout/Generator/Block.php

@@ -261,7 +261,7 @@ protected function getBlockInstance($block, array $arguments = [])
         }
         if (!$block instanceof \Magento\Framework\View\Element\AbstractBlock) {
             throw new \Magento\Framework\Exception\LocalizedException(
-                new \Magento\Framework\Phrase('Invalid block type: %1', [$block]),
+                new \Magento\Framework\Phrase('Invalid block type: %1', [is_object($block) ? get_class($block) : (string) $block]),
                 $e
             );
         }