My control panel no longer displays on the double click window after the update to iOS12.

Many things have changed in iOS 12.

You might want to download the new iPad Users Guide for iOS 12 to famlairise yourself with how iOS 12 works.

With the new iOS 12, you make the Control Panel visible by swiping down from the top right corner of the iPad's screen, now.

Tap on the link below.

iPad User Guide for iOS 12 by Apple Inc.

https://itunes.apple.com/us/book/ipad-user-guide-for-ios-12/id1420662722?mt=11

Also, if you want to access the control panel from within an app, there is a toggle button to toggle ON, in the Settings app, to allow the Control Panel to appear within an active app

The way you bring up the Dock now works a little differently too, in iOS 12, also.

You do a slower short swipe up, then pause when you see the Dock then take your finger off of the screen.

In addition, to bring up the App Switcher, now, you swipe up to the center of the screen, hold for a second, or two, then lift your finger off of the screen.

Lots of new features and things to discover iOS 12.

Control Centre has moved in iOS 12, you now swipe down from the top right-hand edge of the screen