luci-base: dispatcher: fix null access on dispatching unknown urls

Signed-off-by: Jo-Philipp Wich <jo@mein.io>
(cherry picked from commit 4d28c390ab181636b101a17203b2db3391784687)

modules/luci-base/luasrc/dispatcher.lua

@@ -920,7 +920,9 @@ function dispatch(request)
 			return
 		end
 
-		page.readonly = not perm
+		if page then
+			page.readonly = not perm
+		end
 	end
 
 	local action = (page and type(page.action) == "table") and page.action or {}