fix: Restore ability to have empty control template

Author: dr1rrb

src/Uno.UI/UI/Xaml/Controls/Control/Control.cs

@@ -147,6 +147,11 @@ internal View TemplatedRoot
 			get { return _templatedRoot; }
 			set
 			{
+				if (_templatedRoot == value)
+				{
+					return;
+				}
+
 				CleanupView(_templatedRoot);
 
 				UnregisterSubView();

src/Uno.UI/UI/Xaml/FrameworkTemplate.cs

@@ -70,7 +70,7 @@ public FrameworkTemplate(object? owner, FrameworkTemplateBuilder? factory)
 		/// instance that has been detached from its parent may be reused at any time.
 		/// If a control needs to be the owner of a created instance, it needs to use <see cref="LoadContent"/>.
 		/// </remarks>
-		internal View LoadContentCached() => FrameworkTemplatePool.Instance.DequeueTemplate(this);
+		internal View? LoadContentCached() => FrameworkTemplatePool.Instance.DequeueTemplate(this);
 
 		/// <summary>
 		/// Manually return an unused template root created by <see cref="LoadContentCached"/> to the pool.

src/Uno.UI/UI/Xaml/FrameworkTemplatePool.cs

@@ -165,11 +165,11 @@ private void Scavenge(bool isManual)
 		/// normally you shouldn't need to call this method. It may be useful in advanced memory management scenarios.</remarks>
 		public static void Scavenge() => Instance.Scavenge(true);
 
-		internal View DequeueTemplate(FrameworkTemplate template)
+		internal View? DequeueTemplate(FrameworkTemplate template)
 		{
 			var list = GetTemplatePool(template);
 
-			View instance;
+			View? instance;
 
 			if (list.Count == 0)
 			{
@@ -183,7 +183,7 @@ internal View DequeueTemplate(FrameworkTemplate template)
 					this.Log().Debug($"Creating new template, id={GetTemplateDebugId(template)} IsPoolingEnabled:{IsPoolingEnabled}");
 				}
 
-				instance = template.LoadContent() ?? new Grid();
+				instance = template.LoadContent();
 
 				if (IsPoolingEnabled && instance is IFrameworkElement)
 				{
@@ -208,7 +208,7 @@ internal View DequeueTemplate(FrameworkTemplate template)
 			}
 
 #if USE_HARD_REFERENCES
-			if (IsPoolingEnabled)
+			if (IsPoolingEnabled && instance is {})
 			{
 				_activeInstances.Add(instance);
 			}