When creating a new Contract (object NewContract) you should add the association Contract_Klant to that object specifying $Klant as the object for that association.
Hi Ynse,
Yes if you have selected to invoke a microflow when user clicks on new button (to create new contractor) then within the microflow you have to pass the paramater of Klant and once you create new object for contractor you have to associate it with Klant (as show in the screen shot below).
Not sure if you have already completed this but if not then it is worth going through Mendix introduction course that has good videos which explains you everything including your issue in section 11 (As a Customer I want to be able to Request a Flight Booking (Create a Request)).
Hope this helps!
Cheers,
Mohammed Siddiqui