Failed to find or create execution context for description <IBCocoaTouchPlatformToolDescription: 0x7fa8bad9a6f0>
Restarting the CoreSimulatorService solved the issue on my machine.
sudo killall -9 com.apple.CoreSimulator.CoreSimulatorService
I had the same problem after I upgrade my Xcode to version 10.3.
I just restart my Mac and problem solved!
Hope this would work with you too :)
Mine happened when updating to Xcode 10.3
X-code helper didn't get the permission to control the computer.
This is what i did to solve the problem
System Preference > Security&Privacy > privacy tab > accessibilitygive permission for 'Xcode Helper'
Restart X-code and kill all simulators like @riik stated
sudo killall -9 com.apple.CoreSimulator.CoreSimulatorService