After setting up Appium and Nativescript-dev-appium for end-to-end testing in your Nativescript app, you may encounter this error when running your tests for the first time:
TypeError [ERR_INVALID_ARG_TYPE]: The "pid" argument must be of type number. Received type undefined
This happens due to problems with Appium during installation by npm, causing the Appium server to be unable to start and hence being unable to find the process ID. The fix for this is simple - you have to reinstall Appium individually:
npm uninstall appium npm install -g appium
Try running your tests again and they should be successful now.