This can happen in weird instances where cleanup of the app doesn't occur. When it does, the error should give steps for clearing the port, e.g.:
Port 2600 is already occupied. To start Joystick on this port, clear it and try again.
Steps to clear (in your terminal):
1. Run lsof -i :2600 in your terminal.
2. Copy the process ID for the node process running on that port.
3. Run kill <processId> in your terminal.
Repeat the above process for port 2601 to clear Joystick's HMR server.
This can happen in weird instances where cleanup of the app doesn't occur. When it does, the error should give steps for clearing the port, e.g.: