Tweake IMasterDetailPage implementations

inabox.wpf/MasterDetailPanel/IMasterDetailPage.cs

@@ -4,10 +4,14 @@ using InABox.DynamicGrid;
 
 namespace InABox.Wpf;
 
-public interface IMasterDetailPage<TMaster>
+public interface IMasterDetailPage
 {
     DynamicTabItem Tab { get; set; }
+}
 
+public interface IMasterDetailPage<TMaster> : IMasterDetailPage
+{
+    
     Dictionary<string, object[]>? Selected();
 
     IDataModelSource Refresh(TMaster? entity);

inabox.wpf/MasterDetailPanel/MasterDetailPanel.cs

@@ -97,7 +97,7 @@ public abstract class MasterDetailPanel<TMaster, TMasterGrid, TSettings> : Maste
         
     }
     
-    protected void CreatePage<TPage>(Func<bool> isAllowed, string caption) 
+    protected TPage? CreatePage<TPage>(Func<bool> isAllowed, string caption) 
         where TPage : class, IMasterDetailPage<TMaster>
     {
         if (isAllowed())
@@ -105,14 +105,19 @@ public abstract class MasterDetailPanel<TMaster, TMasterGrid, TSettings> : Maste
             var header = new DynamicTabItem() { Header = caption };
             _tabControl.Items.Add(header);
             if (Activator.CreateInstance(typeof(TPage), header) is TPage page)
+            {
                 _pages.Add(page);
+                return page;
+            }
         }
+
+        return null;
     }
 
-    protected void CreatePage<TPage>(string caption)
+    protected TPage? CreatePage<TPage>(string caption)
         where TPage : class, IMasterDetailPage<TMaster>
     {
-        CreatePage<TPage>(() => true, caption);
+        return CreatePage<TPage>(() => true, caption);
     }
 
     public void Shutdown(CancelEventArgs? cancel)