Just updated to macOS Mojave and then from Xcode 9 to Xcode 10 (via App Store). But when I start Xcode 10, it starts for a second and then immediately closes, no error messages or crash report dialogs are shown.
I opened the Console app and saw some log messages that might be related:
CODE SIGNING: cs_invalid_page(0x1048a3000): p=6061[Xcode] final status 0x23006a00, denying page sending SIGKILL
CODE SIGNING: process 6061[Xcode]: rejecting invalid page at address 0x1048a3000 from offset 0x282000 in file "/Applications/Xcode.app/Contents/SharedFrameworks/DVTKit.framework/Versions/A/DVTKit" (cs_mtime:1536705076.0 == mtime:1536705076.0) (signed:1 validated:1 tainted:1 nx:0 wpmapped:0 dirty:0 depth:0)
I solved it by deleting Xcode 10 (and also Xcode 10 beta that I had installed as well). Then reinstalled Xcode 10 from the App Store. Also restarted computer. Then it worked.