Reset Xcode. Clean, clear module cache, Derived Data and Xcode Caches. You can thank me later.

resetXcode.sh

#!/bin/bash
killall Xcode
xcrun -k
xcodebuild -alltargets clean
rm -rf "$(getconf DARWIN_USER_CACHE_DIR)/org.llvm.clang/ModuleCache"
rm -rf "$(getconf DARWIN_USER_CACHE_DIR)/org.llvm.clang.$(whoami)/ModuleCache"
rm -rf ~/Library/Developer/Xcode/DerivedData/*
rm -rf ~/Library/Caches/com.apple.dt.Xcode/*
open /Applications/Xcode.app

worked for me XCode9.4. I had XCode10 beta installed, but it did not work. Then I installed XCode 9.4 which was working, then I removed Xcode10beta, after that the Xcode 9.4 did not work until I ran resetXcode.sh

If you're removing DerivedData and the ModuleCache xcodebuild -alltargets clean is probably redundant.

Optionally consider adding pkill -int com.apple.CoreSimulator.CoreSimulatorService, which we do between Xcode upgrades to minimize the potential for simulator flakiness.

Worked for me with Xcode 9.4.1.
Also, I replaced the last line in script with

TARGET=(*.xcworkspace)
if [ $TARGET == "*.xcworkspace" ]; then
    TARGET=(*.xcodeproj)
    if [ $TARGET == "*.xcodeproj" ]; then
        TARGET=""
    fi
fi
if [ $TARGET != "" ]; then
    open -a "/Applications/Xcode.app" "$TARGET"
else
    echo *** Xcode workspace or project not found
fi

Very nice, fixed an issue I and others had after a react-native-firebase release
invertase/react-native-firebase#2269
thank you!

I created a shell script (tested on bash or zsh) to do this inside any Xcode workspace or project folder.
Check its gist's here.

Life saver!

Great gist!

Please welcome my Apple script which works just from spotlight (⌘+Space)