unable to edit theme php files – Download Manager Plugin

ERROR: Unable to communicate back with site to check for fatal errors, so the PHP change was reverted. You will need to upload your PHP file change by some other means, such as by using SFTP.

This is caused by the Download Manager Plugin. I deactivate the plugin and I can edit my theme’s php files directly inside the dashboard of WordPress. When I click the UPDATE FILE button, it saves my changes. However, if I activate the Download Manager plugin, it causes the above error.