Issue #3531513 by grimreaper: Fatal error with 11.2

Closes #3531513

src/HookHandler/PreprocessViewsViewTable.php

@@ -85,7 +85,8 @@ class PreprocessViewsViewTable implements ContainerInjectionInterface {
 
           $columnContent = '';
           foreach ($column['content'] as $content) {
-            $columnContent .= $this->renderer->render($content['separator']) . $this->renderer->render($content['field_output']);
+            $columnContent .= isset($content['separator']) ? $this->renderer->render($content['separator']) : '';
+            $columnContent .= isset($content['field_output']) ? $this->renderer->render($content['field_output']) : '';
           }
 
           $variables['rows'][$rowKey]['columns'][$columnKey]['preparedContent'] = [