Solution:
Global keyword should be inside the function you want to use it in, like so:
public function get_current_formvalues() {
global $wp_session;
return $wp_session['formvalues'];
}
And also add it in the set method
public function set_formvalues() {
global $wp_session;
//Default values for form
The way it is currently written, $wp_session
is not availble when called. You can read more on http://php.net/manual/en/language.variables.scope.php