I’ve found the solution for this problem. What I did was to place inside the parentheses of the functions the value ($context = ‘view’).
So, for each function I placed this value inside the parentheses in this way:
public function get_price( $context = ‘view’ )
public function get_sale_price( $context = ‘view’ )
public function get_regular_price( $context = ‘view’ )
public function get_weight( $context = ‘view’ )
I only added this value for each function, nothing more and everything works fine so.