iPhone X keeps restarting randomly on its own after updating the software to ios 12 and till 12.1.3

Step 1: Do you have a back up for the device?

No? Please back up the device using a computer if possible.

Yes? See Step 2.

Step 2: Set iPhone X into DFU mode, by hitting: Volume Up then Volume Down then Power.

Plug the device into a computer. Hold the power button down until the Apple logo appears and keep holding until the device is recognized in iTunes as a device in restore mode. Select Restore and restore device as normal. I personally recommend using the device as new for a few days see if the issue is still there.

If the issue still persists there is an issue with the hardware on the iPhone itself.

Check to see if your device is still under limited warranty by going to checkcoverage.apple.com

1. Restart your iPhone: Try this first. Simply restart your device to fix this problem. This is a very simple process. You can shut down your device by going to Settings > General > Shut Down. You can also power off your iPhone by (a) (if it is an iPhone X) pressing and holding the side and one of the volume buttons (up or down); (b) (if it is another iPhone models) pressing and holding the side button.
2. Force close the Phone app and restart: You can close the Phone app and restart again. Do not worry, this will not delete anything. Here is how:
   1. iPhone X: Unlock your device and swipe up. Now you will see the most recently used apps. Find the Phone app, and touch and hold the up and press the red minus (-) icon to close. Then tap the Phone icon and open the Phone app. 
   2. iPhone 8 and earlier models: Double press the home button. Then find the Phone app and swipe up on the Phone app card to close it.