kb(Dialog): Add KB for null DialogFactory

Author: dimodi

components/dialog/predefined.md

@@ -37,6 +37,8 @@ The `DialogFactory` cascading parameter can only be consumed inside child Razor
 * To show predefined dialogs in `MainLayout.razor`, [add the `TelerikRootComponent` in a parent layout]({%slug rootcomponent-overview%}#using-telerikrootcomponent).
 * When [using UI for Blazor in ASP.NET apps]({%slug telerik-blazor-in-asp-net-app%}), an additional child component is needed.
 
+If the `TelerikRootComponent` is not available, or if it's at the wrong place in the app, [the `DialogFactory` will trigger a `NullReferenceException`]({%slug dialog-kb-dialogfactory-null%}).
+
 ### Using await
 
 The `DialogFactory` methods must be awaited. Do not use them with [discard variables (`_`)](https://learn.microsoft.com/en-us/dotnet/csharp/fundamentals/functional/discards). If you don't need to await the user response, then [use the `<TelerikDialog>` component]({%slug dialog-overview%}) declaratively. 

knowledge-base/dialog-dialogfactory-null.md

@@ -0,0 +1,60 @@
+---
+title: DialogFactory is Null
+description: How to troubleshoot, fix and resolve a System.NullReferenceException Object reference not set to an instance of an object, which occurs when the TelerikRootComponent or the Telerik Blazor DialogFactory for predefined dialogs are not setup correctly.
+type: troubleshooting
+page_title: Telerik Blazor DialogFactory is Null
+slug: dialog-kb-dialogfactory-null
+tags: telerik, blazor, dialog, dialogfactory
+ticketid: 1674088, 1673922, 1661511
+res_type: kb
+---
+
+## Environment
+
+<table>
+    <tbody>
+        <tr>
+            <td>Product</td>
+            <td>
+                Dialog for Blazor, <br />
+                RootComponent for Blazor
+            </td>
+        </tr>
+    </tbody>
+</table>
+
+## Description
+
+The following exception may occur when calling the `AlertAsync()`, `ConfirmAsync()`, or `PromptAsync()` methods of the [Telerik Blazor `DialogFactory`]({%slug dialog-predefined%}):
+
+`System.NullReferenceException: Object reference not set to an instance of an object`
+
+The following exception may occur on initial page load:
+
+`InvalidOperationException: Cannot provide a value for property '...' on type '...'. There is no registered service of type 'Telerik.Blazor.DialogFactory'`
+
+## Cause
+
+The null reference exception occurs [if the `DialogFactory` `CascadingParameter` was not populated by the `TelerikRootComponent`]({%slug dialog-predefined%}#telerikrootcomponent-dependency). This in turn indicates one of the following:
+
+* The [`TelerikRootComponent`]({%slug rootcomponent-overview%}) is missing in the Blazor app.
+* The root component is present, but it's in the same `.razor` file as the `DialogFactory` cascading parameter.
+* The Blazor app is using <a href="https://learn.microsoft.com/en-us/aspnet/core/blazor/components/render-modes?view=aspnetcore-9.0#apply-a-render-mode-to-a-component-definition" target="_blank">**Per Page / Component** Interactity Location</a> and the root component is present, but it's in a static `.razor` component or layout.
+
+The invalid operation exception occurs if the app tries to inject the `DialogFactory` as a dependency injection (service), which is not the intended way to use the predefined Telerik dialogs.
+
+## Solution
+
+The following technical requirements ensure correct `DialogFactory` setup:
+
+* [Place a `TelerikRootComponent` in a layout `.razor` file]({%slug rootcomponent-overview%}#using-telerikrootcomponent) in the app when using the application types below. If the `DialogFactory` must be avalable in the layout file itself, then [place the `TelerikRootComponent` in a parent layout file]({%slug getting-started/what-you-need%}#optimal-telerikrootcomponent-usage).
+    * [Blazor Web App]({%slug getting-started/web-app%}) with <a href="https://learn.microsoft.com/en-us/aspnet/core/blazor/components/render-modes?view=aspnetcore-9.0#apply-a-render-mode-to-the-entire-app" target="_blank">**Global** Interactivity Location</a>.
+    * [WebAssembly standalone app]({%slug getting-started/client-side%})
+    * [Blazor Hybrid app]({%slug getting-started/hybrid-blazor%})
+* When using a Blazor Web App with <a href="https://learn.microsoft.com/en-us/aspnet/core/blazor/components/render-modes?view=aspnetcore-9.0#apply-a-render-mode-to-a-component-definition" target="_blank">**Per Page / Component** Interactity Location</a>, [place the `TelerikRootComponent` inside an interactive Razor component]({%slug rootcomponent-percomponent%}), which is a parent of the component that consumes the `DialogFactory`.
+* Always [consume the `DialogFactory` as a `[CascadingParameter]`]({%slug dialog-predefined%}), and not as a dependency injection with `@inject`.
+
+## See Also
+
+* [Predefined Blazor Dialogs]({%slug dialog-predefined%})
+* [TelerikRootComponent Overview]({%slug rootcomponent-overview%})