After several days of hitting my head against the wall I finally found a solution:  make sure that the GRUB_CMDLINE_LINUX line has the following items:            
I have installed Ubuntu 18.04.1 in XPS 15 9570. And installed Nvidia driver 390. If I switch to Intel (Power saving mode), the system can resume back after suspending. But If I want to use the GPU by switching to NVIDIA (Performance Mode) through "Nvidia X Server Settings" wizard, I am not able to resume the system after suspending, the laptop just hangs and I have to forcefully shut down by pressing the power button. I followed the solutions posted here, but no one is working. Even using Bumblebee is not solving the problem. Can anyone help to resolve the issue? What is the basic reason for hanging?
XPS-15 9570 with Ubuntu 18.04.1 not resuming after suspend