Combobox - Validate not called

Question

Can someone please give me a hint for the combobox use (LrView) in the lightroom sdk 6.0? My validate function is not being called. I don't know why. Anybody a hint what I'm doing wrong? Here is the code:

...
        local validateComboboxValue = function (view, value)
            outputToLog("in validate...")
            if(value ~= "Ja" and value ~= "Nein") then
                outputToLog("wrong value")
                view.value = "Ja"
                return false, "Ja", "Ungültige Eingabe"
            else
                outputToLog("valid value")
                return true, value, ""
            end
        end
    ...
        f:combo_box {
            font = "<system>",
            title = "enableSharing",
            enabled = bind 'enableSharingEnabled',
            items = {
                "Ja",
                "Nein"
            },
            value = bind 'sharingEnabled',
            width = LrView.share "label_width",
            immediate = true,
            validate = validateComboboxValue,
        },
    ...

I'm using Lightroom on a Windows 10 machine.