Check for correct state on attach/detach to tree.

Make sure that if we're attached to a logical tree, we can invoke
DetachedFromLogicalTree when Parent is set to null. This also has a side
effect in that Parent will now be non-null when DetachedFromLogicalTree
is called.

src/Perspex.Base/Perspex.Base.csproj

@@ -63,6 +63,7 @@
     <Compile Include="Metadata\DependsOnAttribute.cs" />
     <Compile Include="Metadata\ContentAttribute.cs" />
     <Compile Include="PerspexDisposable.cs" />
+    <Compile Include="PerspexInternalException.cs" />
     <Compile Include="PerspexLocator.cs" />
     <Compile Include="Metadata\XmlnsDefinitionAttribute.cs" />
     <Compile Include="PerspexObjectExtensions.cs" />

src/Perspex.Base/PerspexInternalException.cs

@@ -0,0 +1,22 @@
+// Copyright (c) The Perspex Project. All rights reserved.
+// Licensed under the MIT license. See licence.md file in the project root for full license information.
+
+using System;
+
+namespace Perspex
+{
+    /// <summary>
+    /// Exception signifying an internal logic error in Perspex.
+    /// </summary>
+    public class PerspexInternalException : Exception
+    {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="PerspexInternalException"/> class.
+        /// </summary>
+        /// <param name="message">The exception message.</param>
+        public PerspexInternalException(string message)
+            : base(message)
+        {
+        }
+    }
+}

src/Perspex.Controls/Control.cs

@@ -391,22 +391,32 @@ namespace Perspex.Controls
                     throw new InvalidOperationException("The Control already has a parent.");
                 }
 
-                InheritanceParent = parent as PerspexObject;
-                _parent = (IControl)parent;
+                if (_isAttachedToLogicalTree)
+                {
+                    var oldRoot = FindStyleRoot(old);
 
-                var root = FindStyleRoot(old);
+                    if (oldRoot == null)
+                    {
+                        throw new PerspexInternalException("Was attached to logical tree but cannot find root.");
+                    }
 
-                if (root != null)
-                {
-                    var e = new LogicalTreeAttachmentEventArgs(root);
+                    var e = new LogicalTreeAttachmentEventArgs(oldRoot);
                     OnDetachedFromLogicalTree(e);
                 }
 
-                root = FindStyleRoot(this);
+                InheritanceParent = parent as PerspexObject;
+                _parent = (IControl)parent;
 
-                if (root != null)
+                if (_parent is IStyleRoot || _parent?.IsAttachedToLogicalTree == true)
                 {
-                    var e = new LogicalTreeAttachmentEventArgs(root);
+                    var newRoot = FindStyleRoot(this);
+
+                    if (newRoot == null)
+                    {
+                        throw new PerspexInternalException("Parent is atttached to logical tree but cannot find root.");
+                    }
+
+                    var e = new LogicalTreeAttachmentEventArgs(newRoot);
                     OnAttachedToLogicalTree(e);
                 }