Make sure AttachedToLogicalTree is only raised once.

And add a check to make sure control is attached to logical tree before
detach - I *think* this should always be the case, though there may be
an edge case which triggers this exception, in which case do the same
for DetachedFromLogicalTree as we've done for AttachedToLogicalTree
here.

src/Perspex.Controls/Control.cs

@@ -406,19 +406,28 @@ namespace Perspex.Controls
         /// </remarks>
         protected virtual void OnAttachedToLogicalTree(LogicalTreeAttachmentEventArgs e)
         {
-            if (_nameScope == null)
+            // This method can be called when a control is already attached to the logical tree
+            // in the following scenario:
+            // - ListBox gets assigned Items containing ListBoxItem
+            // - ListBox makes ListBoxItem a logical child
+            // - ListBox template gets applied; making its Panel get attached to logical tree
+            // - That AttachedToLogicalTree signal travels down to the ListBoxItem
+            if (!_isAttachedToLogicalTree)
             {
-                _nameScope = NameScope.GetNameScope(this) ?? ((Control)Parent)?._nameScope;
-            }
+                if (_nameScope == null)
+                {
+                    _nameScope = NameScope.GetNameScope(this) ?? ((Control)Parent)?._nameScope;
+                }
 
-            if (Name != null)
-            {
-                _nameScope?.Register(Name, this);
-            }
+                if (Name != null)
+                {
+                    _nameScope?.Register(Name, this);
+                }
 
-            _isAttachedToLogicalTree = true;
-            PerspexLocator.Current.GetService<IStyler>()?.ApplyStyles(this);
-            AttachedToLogicalTree?.Invoke(this, e);
+                _isAttachedToLogicalTree = true;
+                PerspexLocator.Current.GetService<IStyler>()?.ApplyStyles(this);
+                AttachedToLogicalTree?.Invoke(this, e);
+            }
 
             foreach (var child in LogicalChildren.OfType<Control>())
             {
@@ -436,6 +445,11 @@ namespace Perspex.Controls
         /// </remarks>
         protected virtual void OnDetachedFromLogicalTree(LogicalTreeAttachmentEventArgs e)
         {
+            if (!_isAttachedToLogicalTree)
+            {
+                throw new Exception("Logic error: Control is not attached to logical tree");
+            }
+
             if (Name != null)
             {
                 _nameScope?.Unregister(Name);