Error messages when activating

Hello:
I am running WordPress 4.9.8 on a Windows machine. I am getting the following error when I activate the WordPress MooTools Tree Creator plugin:

The plugin generated 360 characters of unexpected output during activation. If you notice “headers already sent” messages, problems with syndication feeds or other issues, try deactivating or removing this plugin.

Click here for screenshot

Additionally, the following warning appears in all WP_Admin screens, no matter whether WP_DEBUG is true or false:

Warning: Declaration of Mootree_walker::start_el(&$output, $page, $depth, $args, $current_page = 0) should be compatible with Walker_Page::start_el(&$output, $page, $depth = 0, $args = Array, $current_page = 0) in C:\Bitnami\wordpress-4.9.8-0\apps\wordpress\htdocs\wp-content\plugins\wp-moo-tree\tree-shortcode.php on line 0

This plugin is causing degradation of basic functionality of WordPress. For instance, the items in the Media Library do not show:

Click here for screenshot

There is no error message output to my browser console; the media files simply do not show.

I’m not sure whether this is one bug or many, as I’m not sure they’re related. I’m guessing that they are. But I have tried deactivating every other plugin on my site, and both these problems persist.