Clear Relationship Field

Hi Guys, 
I have a form with date, time and Location. 

Location in a Relationship Field (N:1)

With my Button I can sent everything directly to the table. After that the fields date and time are cleaned.

I try to use the same command for the N:1 field but it wasn't working. Do you have an idea how I can clean the field after I hid the button?

if Datum = null then
    alert("Fehler: Es wurde kein Betrag eingetragen!")
else
    let my := this;
    let i := (create Veranstaltungen);
    do as server
        i.(Datum := my.Datum);
        i.(Uhrzeit := my.Uhrzeit);
        i.(Location := my.Locations);
        my.(Datum := null);
        my.(Uhrzeit := null);
        openRecord(this)
    end
end