fix exception on getting formatStr, also put that logic in an extension method to be consistent

Author: adelikat

TASVideos/Extensions/ViewDataDictionaryExtensions.cs

@@ -51,6 +51,9 @@ public static void SetNavigation(this ViewDataDictionary viewData, int id, strin
 		viewData["NavigationFmtStr"] = fmtStr;
 	}
 
+	public static string GetNavigationFormatStr(this ViewDataDictionary viewData)
+		=> viewData["NavigationFmtStr"] as string ?? "";
+
 	public static string ActivePageClass(this ViewDataDictionary viewData, string page)
 	{
 		var activePage = viewData["ActivePage"] as string;

TASVideos/Pages/Shared/_Layout.cshtml

@@ -89,7 +89,7 @@
 		var heading = ViewData.GetHeading();
 		int? prev = null;
 		int? next = null;
-		string? fmtStr = ViewData["NavigationFmtStr"] as string;
+		string fmtStr = ViewData.GetNavigationFormatStr();
 		if (User.Has(PermissionTo.CatalogMovies))
 		{
 			var id = ViewData.Int("NavigationId");