Fix SettingsViewModelTest.CannotAddChildElementToFormatElement.

Tests/ViewModels/SettingsViewModelTest.cs

@@ -58,7 +58,7 @@ namespace Tests.ViewModels
             parent.IsSelected = true;
             Assert.IsTrue(_settingsVM.AddChildElementCommand.CanExecute(null));
             FormatElementViewModel child = new FormatElementViewModel(new Items());
-            parent.AddChildElement(child);
+            _settingsVM.AddChildElementViewModel(parent, child);
             parent.IsSelected = false;
             child.IsSelected = true;
             Assert.IsFalse(_settingsVM.AddChildElementCommand.CanExecute(null));