Fix date/time picker with overlay popups.

When using overlay popups, a measure won't be done immediately on open, so the presenter template won't be set. Force a layout pass in this case. Also add a null check in `GetOffsetForPopup` just in case it does happen by some chance elsewhere.

Fixes #7632

src/Avalonia.Controls/DateTimePickers/DatePicker.cs

@@ -4,6 +4,7 @@ using Avalonia.Controls.Shapes;
 using Avalonia.Controls.Templates;
 using Avalonia.Data;
 using Avalonia.Interactivity;
+using Avalonia.Layout;
 using System;
 using System.Collections.Generic;
 using System.Globalization;
@@ -410,6 +411,11 @@ namespace Avalonia.Controls
             _popup.PlacementConstraintAdjustment = Primitives.PopupPositioning.PopupPositionerConstraintAdjustment.SlideY;
             _popup.IsOpen = true;
 
+            // Overlay popup hosts won't get measured until the next layout pass, but we need the
+            // template to be applied to `_presenter` now. Detect this case and force a layout pass.
+            if (!_presenter.IsMeasureValid)
+                (VisualRoot as ILayoutRoot)?.LayoutManager?.ExecuteInitialLayoutPass();
+
             var deltaY = _presenter.GetOffsetForPopup();
 
             // The extra 5 px I think is related to default popup placement behavior

src/Avalonia.Controls/DateTimePickers/DatePickerPresenter.cs

@@ -537,8 +537,11 @@ namespace Avalonia.Controls
 
         internal double GetOffsetForPopup()
         {
+            if (_monthSelector is null)
+                return 0;
+
             var acceptDismissButtonHeight = _acceptButton != null ? _acceptButton.Bounds.Height : 41;
-            return -(MaxHeight - acceptDismissButtonHeight) / 2 - (_monthSelector!.ItemHeight / 2);
+            return -(MaxHeight - acceptDismissButtonHeight) / 2 - (_monthSelector.ItemHeight / 2);
         }
     }
 }

src/Avalonia.Controls/DateTimePickers/TimePicker.cs

@@ -3,6 +3,7 @@ using Avalonia.Controls.Primitives;
 using Avalonia.Controls.Shapes;
 using Avalonia.Controls.Templates;
 using Avalonia.Data;
+using Avalonia.Layout;
 using System;
 using System.Globalization;
 
@@ -267,6 +268,11 @@ namespace Avalonia.Controls
             _popup.PlacementConstraintAdjustment = Primitives.PopupPositioning.PopupPositionerConstraintAdjustment.SlideY;
             _popup.IsOpen = true;
 
+            // Overlay popup hosts won't get measured until the next layout pass, but we need the
+            // template to be applied to `_presenter` now. Detect this case and force a layout pass.
+            if (!_presenter.IsMeasureValid)
+                (VisualRoot as ILayoutRoot)?.LayoutManager?.ExecuteInitialLayoutPass();
+
             var deltaY = _presenter.GetOffsetForPopup();
 
             // The extra 5 px I think is related to default popup placement behavior

src/Avalonia.Controls/DateTimePickers/TimePickerPresenter.cs

@@ -256,8 +256,11 @@ namespace Avalonia.Controls
 
         internal double GetOffsetForPopup()
         {
+            if (_hourSelector is null)
+                return 0;
+
             var acceptDismissButtonHeight = _acceptButton != null ? _acceptButton.Bounds.Height : 41;
-            return -(MaxHeight - acceptDismissButtonHeight) / 2 - (_hourSelector!.ItemHeight / 2);
+            return -(MaxHeight - acceptDismissButtonHeight) / 2 - (_hourSelector.ItemHeight / 2);
         }
     }
 }