Fatal Error when checking for stock for multivariation products

When you enable option «Respect "Commerce Stock" module» it causes fatal errors for product with more than one variation. The thing is the module gets product id from "#value" paremeter of hidden form element «product_id» but in case of several variations instead of it we have a select with «#default_value» paremeter. I do not include check for existance of «product_id» input itself because it's already solved in this issue.