Steps to reproduce the issue

I've Joomla 3.9.1 version. And I'm facing an interesting issue. I am not able to edit the "Editor - TinyMCE" plugin from the back-end. I've checked that it's not locked. Following is the URL which takes forever to load.

/administrator/index.php?option=com_plugins&task=plugin.edit&extension_id=412

I have tried to debug code and found out the issue in the following code in "administrator/components/com_plugins/views/plugin/tmpl/edit.php" file.

<?php
				$this->fieldset = 'basic';
				$html = JLayoutHelper::render('joomla.edit.fieldset', $this);
				echo $html ? '<hr />' . $html : '';
?>

If I comment the "$html = JLayoutHelper::render('joomla.edit.fieldset', $this);" line, it loads the edit plugin interface, but blank page with generic options. If I change $this->fieldset from "basic' to 'advanced' it also works.

So, there's something with 'basic' option.

Can anyone help on this?

Expected result

Actual result

System information (as much as possible)

Additional comments