Fixed an error that appears when trying to delete an instance that is already deleted. (#109)

* Prevent deleting already removed instances.

* Prehashed empty hash.

* Use go.exists instead of pcall, when detecting non existing instances.

README_API.md

@@ -222,6 +222,13 @@ IMPORTANT! If you provide a function you must also make sure to call `monarch.on
 * `fn_or_url` (function) - The function to call or URL to send message to
 
 
+## monarch.has_missing_resources(screen_id)
+Check if a screen has any missing resources. If the screen is loaded by a collection proxy with Load Dynamically this will use `collectionproxy.missing_resources()`. For other cases this function will always return false.
+
+**RETURN**
+* `result` (boolean) - true if the screen has missing resources
+
+
 ## monarch.debug()
 Enable verbose logging of the internals of Monarch.
 

monarch/monarch.lua

@@ -430,8 +430,10 @@ local function unload(screen, force)
 	elseif screen.factory then
 		log("unload() factory", screen.id)
 		for id, instance in pairs(screen.factory_ids) do
+			if go.exists(instance) then
 				go.delete(instance)
 			end
+		end
 		screen.factory_ids = nil
 		if screen.auto_preload and not force then
 			screen.loaded = false
@@ -1374,8 +1376,10 @@ function M.on_post(id, fn_or_url)
 	end
 end
 
+local empty_hash = hash("")
+
 local function url_to_key(url)
-	return (url.socket or hash("")) .. (url.path or hash("")) .. (url.fragment or hash(""))
+	return (url.socket or empty_hash) .. (url.path or empty_hash) .. (url.fragment or empty_hash)
 end