Problem with date picker showing (main thread issue?)

Working with an AppleScript file from https://github.com/bat-tomr/dialog-node ...

# 06/04/16 09:35:02
# Author: Shane Stanley
# Adapted by Christopher Stone

use AppleScript version "2.4"
use scripting additions
use framework "Foundation"
use framework "AppKit"

my datePicker()

on datePicker()
    set theApp to path to frontmost application as text

    if not (current application's NSThread's isMainThread()) as boolean then
        display alert "This script must be run from the main thread." buttons {"Cancel"} as critical
        error number -128
    end if
    -- create a view
    set theView to current application's NSView's alloc()'s initWithFrame:(current application's NSMakeRect(0, 0, 100, 175))
    -- create date picker
    set datePicker to current application's NSDatePicker's alloc()'s initWithFrame:(current application's NSMakeRect(0, 0, 100, 100))
    -- set style: choices are NSTextFieldAndStepperDatePickerStyle, NSClockAndCalendarDatePickerStyle, or NSTextFieldDatePickerStyle
    #datePicker's setDatePickerStyle:(current application's NSClockAndCalendarDatePickerStyle)
    datePicker's setDatePickerStyle:(current application's NSTextFieldAndStepperDatePickerStyle)
    #datePicker's setDatePickerStyle:(current application's NSTextFieldDatePickerStyle)
    -- set elements: choices include NSHourMinuteDatePickerElementFlag, NSHourMinuteSecondDatePickerElementFlag, NSTimeZoneDatePickerElementFlag, NSYearMonthDatePickerElementFlag, and NSEraDatePickerElementFlag
    datePicker's setDatePickerElements:((current application's NSYearMonthDayDatePickerElementFlag) + (current application's NSHourMinuteSecondDatePickerElementFlag as integer))

    -- set initial date
    #datePicker's setDateValue:(current application's NSDate's |date|())
    --set theCal var to a new(empty) instance of the calendar
    set theCal to current application's class "NSCalendar"'s currentCalendar()
    -- unpack the components of theCal to a variable
    set theComponents to theCal's components:254 fromDate:(current application's NSDate's |date|())
    theComponents's setSecond:0
    theComponents's setMinute:0
    theComponents's setHour:12
    theComponents's setYear:2015
    theComponents's setMonth:4
    theComponents's setDay:1
    datePicker's setDateValue:(theCal's dateFromComponents:theComponents)

    -- get the size it needs
    set theSize to datePicker's fittingSize()
    --resize the picker and view accordingly
    theView's setFrameSize:theSize
    datePicker's setFrameSize:theSize
    -- add the picker to the view
    theView's setSubviews:{datePicker}
    -- create an alert
    set theAlert to current application's NSAlert's alloc()'s init()
    -- set up alert
    tell theAlert
        its setMessageText:"Pick a date and time"
        its setInformativeText:"Any date"
        its addButtonWithTitle:"OK"
        its addButtonWithTitle:"Cancel"
        its setAccessoryView:theView
    end tell

    -- show alert in modal loop
    set returnCode to theAlert's runModal()
    if returnCode = (current application's NSAlertSecondButtonReturn) then error number -128
    -- retrieve date
    #set theDate to datePicker's dateValue() as date
    # Shane Stanley
    # http://macscripter.net/viewtopic.php?pid=119633#p119633
    set theDate to datePicker's dateValue()
    #set theCal to current application's class "NSCalendar"'s currentCalendar()
    -- unpack the components of theDate to a variable
    set theComponents to theCal's components:254 fromDate:theDate
    set theDate to current date
    set day of theDate to 1 -- important
    set seconds of theDate to theComponents's |second|()
    set year of theDate to theComponents's |year|()
    set month of theDate to theComponents's |month|()
    set day of theDate to theComponents's |day|()
    set hours of theDate to theComponents's hour()
    set minutes of theDate to theComponents's minute()

    return (theDate as text)
    #return (theDate)
end datePicker

The date picker does not display (at least on Mojave). It does display, however, if I put a display alert before runModal (but not otherwise).

I understand this may have to do with a need for performSelectorOnMainThread in latter Mac OSX versions, but if that is the case, I was not able to add it such that the code would work. Any ideas?

Here you go... Not much to say really. I basically just rewrote it because I can't stand messy code:

# 06/04/16 09:35:02
# Author: Shane Stanley
# Adapted by Christopher Stone
# Fixed & Rewritten by CJK
--------------------------------------------------------------------------------
use framework "AppKit"
use scripting additions

property this : a reference to the current application
property nil : a reference to missing value
property _1 : a reference to reference

property NSAlert : a reference to NSAlert of this
property NSDatePicker : a reference to NSDatePicker of this
property NSView : a reference to NSView of this

property NSAlertSecondButtonReturn : 1001
property NSHourMinuteSecondDatePickerElementFlag : 14
property NSTextFieldAndStepperDatePickerStyle : 0
property NSYearMonthDayDatePickerElementFlag : 224
--------------------------------------------------------------------------------
property date : missing value
--------------------------------------------------------------------------------
on run
    its performSelectorOnMainThread:("showDatePicker:") withObject:{¬
        NSTextFieldAndStepperDatePickerStyle, ¬
        NSYearMonthDayDatePickerElementFlag + ¬
        NSHourMinuteSecondDatePickerElementFlag} ¬
        waitUntilDone:true

    return my date
end run

on showDatePicker:params
    local params

    set {PickerStyle, PickerElements} to params

    tell (current date) to set ¬
        [dateFrom, day, its month, day, year, time] to ¬
        [it, 1, 4, 1, 2015, 12 * hours + 0 * minutes]

    tell NSDatePicker's alloc()
        initWithFrame_({{0, 0}, {100, 100}})
        setDatePickerStyle_(PickerStyle)
        setDatePickerElements_(PickerElements)
        setDateValue_(dateFrom)
        set fittingSize to fittingSize()
        setFrameSize_(fittingSize)

        set View to NSView's alloc()
        View's initWithFrame:{{0, 0}, {100, 175}}
        View's setFrameSize:fittingSize
        View's addSubview:it

        tell NSAlert's alloc()
            init()
            setMessageText_("Pick a date and time")
            setInformativeText_("Any date")
            addButtonWithTitle_("OK")
            addButtonWithTitle_("Cancel")
            setAccessoryView_(View)

            runModal()
        end tell

        set my date to dateValue() as date
    end tell
end showDatePicker:
---------------------------------------------------------------------------❮END❯

^{System info: AppleScript version: 2.7 System version: 10.13.6}