Export error

when I move plugin to mu-plugins directory, export does not work

Fatal error: Call to a member function display() on a non-object in C:\SERVER\htdocs\iappraisal-wp\web\app\mu-plugins\visual-form-builder\visual-form-builder.php on line 1669

within plugins directory works fine

https://ww.wp.xz.cn/plugins/visual-form-builder/