Fixes U4-1394 so the DefaultTemplate is not completely removed from the list of allowed templates when changed.

src/Umbraco.Core/Models/ContentType.cs

@@ -72,6 +72,12 @@ namespace Umbraco.Core.Models
         /// <param name="template">Default <see cref="ITemplate"/></param>
         public void SetDefaultTemplate(ITemplate template)
         {
+            if (template == null)
+            {
+                DefaultTemplateId = 0;
+                return;
+            }
+
             DefaultTemplateId = template.Id;
             if(_allowedTemplates.Any(x => x != null && x.Id == template.Id) == false)
             {

src/Umbraco.Web/umbraco.presentation/umbraco/settings/EditNodeTypeNew.aspx.cs

@@ -128,6 +128,8 @@ namespace umbraco.settings
                     else
                         dt.RemoveDefaultTemplate();
 
+                    dt.Save();
+
                     bindTemplates();
                 }
                 else

src/umbraco.cms/businesslogic/web/DocumentType.cs

@@ -420,14 +420,11 @@ namespace umbraco.cms.businesslogic.web
             return doc;
         }
 
-        [Obsolete("Deprecated, Use RemoveTemplate() on Umbraco.Core.Models.ContentType", false)]
+        [Obsolete("Deprecated, Use SetDefaultTemplate(null) on Umbraco.Core.Models.ContentType", false)]
         public void RemoveDefaultTemplate()
         {
             _defaultTemplate = 0;
-
-            var template = _contentType.DefaultTemplate;
-            if(template != null)
-                _contentType.RemoveTemplate(template);
+            _contentType.SetDefaultTemplate(null);
         }
 
         public bool HasTemplate()