Ensure Presenter template is applied...

...before updating ContentControl's logical children, as otherwise the
Presenter.Child may be null as UpdateChild has not yet been called.

src/Perspex.Controls/Mixins/ContentControlMixin.cs

@@ -59,6 +59,8 @@ namespace Perspex.Controls.Mixins
 
                     if (presenter != null)
                     {
+                        presenter.ApplyTemplate();
+
                         var logicalChildren = logicalChildrenSelector(sender);
                         var subscription = presenter
                             .GetObservable(ContentPresenter.ChildProperty)

tests/Perspex.Controls.UnitTests/ContentControlTests.cs

@@ -54,7 +54,6 @@ namespace Perspex.Controls.UnitTests
                 root.Child = target;
 
                 target.ApplyTemplate();
-                target.Presenter.UpdateChild();
 
                 styler.Verify(x => x.ApplyStyles(It.IsAny<ContentControl>()), Times.Once());
                 styler.Verify(x => x.ApplyStyles(It.IsAny<Border>()), Times.Once());