I did some more research on this – looks like the plugin is loading WordPress admin.php inside the tinymce dialog window and that in turn tries to execute the admin_init action hooks already registered when the page edit window is loaded. But the tinymce dialog does not have any of the classes (that registered in the first place) loaded which is causing the dialog to crash when trying to execute the admin_init action hook by throwing a file/class/function not found error. I am not sure how good an idea it is to actually do a reload of WordPress admin inside a tinymce dialog. Can cause issues with any plugin which registers itself to admin_init hook inside the Page Edit window, not just OptionTree.