RadTimePicker objects remain in memory because, in the DateTimePicker class, it subscribe  on event Window.Current.SizeChanged but you never unsubscribe. So it generates memory leaks. The control will never be removed by the GC(garbage collector).

Source code:

 /// <summary>
        /// Initializes a new instance of the <see cref="DateTimePicker"/> class.
        /// </summary>
        protected DateTimePicker()
        {
            var languages = new List<string> { this.CalendarLanguage };
            var calendar = new Windows.Globalization.Calendar(languages, this.CalendarIdentifier, ClockIdentifiers.TwelveHour);
            this.calendarValidator = new CalendarValidator(calendar);
            this.selectorUtcValue = DateTime.SpecifyKind(DateTime.Now, DateTimeKind.Utc);
            this.utcValue = this.selectorUtcValue;
            Window.Current.SizeChanged += this.Current_SizeChanged;
        }

I am trying to use the chart control in multiple views: Meaning not just one window. I create several windows from the main window. It is not call "windows". It is a view.

It looks like the chart control fails:

System.Exception: The application called an interface that was marshalled for a different thread. (Exception from HRESULT: 0x8001010E (RPC_E_WRONG_THREAD))
   at Windows.UI.Xaml.Media.SolidColorBrush.get_Color()
   at Telerik.UI.Xaml.Controls.Chart.ContainerVisualsFactory.ChangeBrushesAccordingToAppTheme()
   at Telerik.UI.Xaml.Controls.Chart.ContainerVisualsFactory.PrepareCartesianAxisLineVisual(CartesianAxis axis, ContainerVisual lineContainer, RadRect layoutSlot, AxisType axisType)
   at Telerik.UI.X

Is there a way to fix it ?

to understand the concept of multiple views:

https://docs.microsoft.com/en-us/windows/uwp/design/layout/show-multiple-views

please advise me...

The "Foreground" property does not apply to the RadDatePicker icon .. it only applies to the text even though I followed what this page explains: https://docs.telerik.com/windows-universal/controls/raddatepicker-and-radtimepicker/styling/raddatetimepickers-explicitstyling

My XAML code is the following:

<Custom:RadDatePicker x:Name="FromDatePicker" HorizontalAlignment="Left" ValueChanged="DatePickerValueChange">
                    <Custom:RadDatePicker.Resources>
                        <Style TargetType="Custom:RadDatePicker">
                            <Setter Property="Foreground" Value="Orange"/>
                            <Setter Property="FontSize" Value="15"/>
                            <Setter Property="FontStyle" Value="Italic"/>
                            <Setter Property="Background" Value="Beige"/>
                            <Setter Property="BorderBrush" Value="Orange"/>
                        </Style>
                    </Custom:RadDatePicker.Resources>
                </Custom:RadDatePicker>

Attached image of the properties that only my RadDatePicker takes when running the application ..

Provide component that is able to read and decode codes from image.

Universal barcode.

Make the gauges universal.

This would let us create LOB applications consistent with the Look & Feel of Office 2016 Universal apps.

Please, add the RadMap control to your plan

It is hard to migrate to universal app w/o this one.

Create a Masked Textblock that can be used in Universal Windows Apps.

Provide a universal LiveTileHelper similar to the Windows Phone LiveTileHelper.

Waterfall chart for universal windows apps

Make the RadRadialMenu control available for Windows Phone.

Provide a universal JumpList similar to the Windows Phone JumpList.

Universal uniform grid.

Universal tab control.

Sparkline chart for universal windows apps.

Create a slide view that can be used in Universal Windows Apps.

1. Single thumb,
2. Range,
3. Radial