DatePicker/TimePicker do not work with overlay popups.

[grokys]

Describe the bug

Trying to use a DatePicker/TimePicker when overlay popups is enabled results in a null reference exception.

To Reproduce
Steps to reproduce the behavior:

1. On Windows, add OverlayPopups = true to the Win32PlatformOptions in ControlCatalog.NetCore
2. Try to open a DatePicker/TimePicker

System.NullReferenceException
  HResult=0x80004003
  Message=Object reference not set to an instance of an object.
  Source=Avalonia.Controls
  StackTrace:
   at Avalonia.Controls.TimePickerPresenter.GetOffsetForPopup() in D:\projects\AvaloniaUI\Avalonia\src\Avalonia.Controls\DateTimePickers\TimePickerPresenter.cs:line 260

[grokys]

This is because when using overlay popups, a measure isn't carried out immediately (as is the case with a PopupRoot).

TimePicker opens the popup here and then immediately calls _presenter.GetOffsetForPopup(); which assumes that the template will have already been applied. With overlay popups, this is not the case.