Appium tests fail because 'Selenium error: session is terminated'

[MEDIC LOG Mon, 09 May 2016 18:15:04 GMT] Uncaught exception! Killing server and exiting in 2 seconds...
[MEDIC LOG Mon, 09 May 2016 18:15:04 GMT] Uncaught exception! Killing server and exiting in 2 seconds...
[MEDIC LOG Mon, 09 May 2016 18:15:05 GMT] Appium process exited.
[MEDIC LOG Mon, 09 May 2016 18:15:06 GMT] FATAL: Error: [currentContext()] Error response status: 6,  Selenium error: A session is either terminated or not started
    at exports.newError (/Users/buildbot/slaves/apache/cordova-ios/cordova-medic/node_modules/wd/lib/utils.js:139:13)
    at /Users/buildbot/slaves/apache/cordova-ios/cordova-medic/node_modules/wd/lib/callbacks.js:75:19
    at /Users/buildbot/slaves/apache/cordova-ios/cordova-medic/node_modules/wd/lib/webdriver.js:174:5
    at Request._callback (/Users/buildbot/slaves/apache/cordova-ios/cordova-medic/node_modules/wd/lib/http-utils.js:87:7)
    at Request.self.callback (/Users/buildbot/slaves/apache/cordova-ios/cordova-medic/node_modules/request/request.js:368:22)
    at Request.emit (events.js:110:17)
    at Request.<anonymous> (/Users/buildbot/slaves/apache/cordova-ios/cordova-medic/node_modules/request/request.js:1219:14)
    at Request.emit (events.js:129:20)
    at IncomingMessage.<anonymous> (/Users/buildbot/slaves/apache/cordova-ios/cordova-medic/node_modules/request/request.js:1167:12)
    at IncomingMessage.emit (events.js:129:20)
program finished with exit code 1
elapsedTime=3700.311833