Hi Abutalib shaikh,
It happens because there is a Mendix app running in the background with the port 8040.
While working in your current project port from 8040 to any other like 8090 and try.
App Settings à Configurations à Double click the configuration that is ticked In green colour à Change in admin port.
You can find the other Mendix app running on the background b checking on Task manager tab in your system.
Hope you find it helpful!