Failed to load resource: net::ERR_FILE_NOT_FOUND polyfills.js:3 POST file:///android_asset/www/null net::ERR_FILE_NOT_FOUND

Am getting
file:///android_asset/www/null net::ERR_FILE_NOT_FOUND after opening the app for sometimes in ionic 3 app. I can’t able to find the solution even i changed this file but it doesn’t work this is the path of file:
i made src == null .
if (src.charAt(0) == ‘/’) {
src = src.substring(1);
if(src == null){
launchUrl = “file:///android_asset/www/” + src;
can anybody help me out from this problem.
Remeber :: This problem not comming continuously its comming becz of keeping app open for some time… time