PageClassFieldTest#testSingleSelection is flickering

Error Message

Unable to locate element: ./ul[@id='fields']/li[last()]
For documentation on this error, please visit: https://www.seleniumhq.org/exceptions/no_such_element.html
Build info: version: '3.141.59', revision: 'e82be7d358', time: '2018-11-14T08:17:03'
System info: host: '0a1ad5ac12b8', ip: '172.17.0.2', os.name: 'Linux', os.arch: 'amd64', os.version: '4.19.0-9-amd64', java.version: '1.8.0_242'
Driver info: org.openqa.selenium.firefox.FirefoxDriver
Capabilities {acceptInsecureCerts: true, browserName: firefox, browserVersion: 74.0, javascriptEnabled: true, moz:accessibilityChecks: false, moz:buildID: 20200309095159, moz:geckodriverVersion: 0.26.0, moz:headless: false, moz:processID: 1354, moz:profile: /tmp/rust_mozprofile3LfDK3, moz:shutdownTimeout: 60000, moz:useNonSpecCompliantPointerOrigin: false, moz:webdriverClick: true, pageLoadStrategy: normal, platform: LINUX, platformName: LINUX, platformVersion: 4.19.0-9-amd64, rotatable: false, setWindowRect: true, strictFileInteractability: false, timeouts: {implicit: 0, pageLoad: 300000, script: 30000}, unhandledPromptBehavior: dismiss and notify}
Session ID: 35f14a47-a3d1-4054-9f27-ef11eb3eb81b
*** Element info: {Using=xpath, value=./ul[@id='fields']/li[last()]}

Stacktrace

org.openqa.selenium.NoSuchElementException: 
Unable to locate element: ./ul[@id='fields']/li[last()]
For documentation on this error, please visit: https://www.seleniumhq.org/exceptions/no_such_element.html
Build info: version: '3.141.59', revision: 'e82be7d358', time: '2018-11-14T08:17:03'
System info: host: '0a1ad5ac12b8', ip: '172.17.0.2', os.name: 'Linux', os.arch: 'amd64', os.version: '4.19.0-9-amd64', java.version: '1.8.0_242'
Driver info: org.openqa.selenium.firefox.FirefoxDriver
Capabilities {acceptInsecureCerts: true, browserName: firefox, browserVersion: 74.0, javascriptEnabled: true, moz:accessibilityChecks: false, moz:buildID: 20200309095159, moz:geckodriverVersion: 0.26.0, moz:headless: false, moz:processID: 1354, moz:profile: /tmp/rust_mozprofile3LfDK3, moz:shutdownTimeout: 60000, moz:useNonSpecCompliantPointerOrigin: false, moz:webdriverClick: true, pageLoadStrategy: normal, platform: LINUX, platformName: LINUX, platformVersion: 4.19.0-9-amd64, rotatable: false, setWindowRect: true, strictFileInteractability: false, timeouts: {implicit: 0, pageLoad: 300000, script: 30000}, unhandledPromptBehavior: dismiss and notify}
Session ID: 35f14a47-a3d1-4054-9f27-ef11eb3eb81b
*** Element info: {Using=xpath, value=./ul[@id='fields']/li[last()]}
	at sun.reflect.GeneratedConstructorAccessor38.newInstance(Unknown Source)
	at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
	at java.lang.reflect.Constructor.newInstance(Constructor.java:423)
	at org.openqa.selenium.remote.http.W3CHttpResponseCodec.createException(W3CHttpResponseCodec.java:187)
	at org.openqa.selenium.remote.http.W3CHttpResponseCodec.decode(W3CHttpResponseCodec.java:122)
	at org.openqa.selenium.remote.http.W3CHttpResponseCodec.decode(W3CHttpResponseCodec.java:49)
	at org.openqa.selenium.remote.HttpCommandExecutor.execute(HttpCommandExecutor.java:158)
	at org.openqa.selenium.remote.service.DriverCommandExecutor.execute(DriverCommandExecutor.java:83)
	at org.openqa.selenium.remote.RemoteWebDriver.execute(RemoteWebDriver.java:552)
	at org.openqa.selenium.remote.RemoteWebElement.execute(RemoteWebElement.java:285)
	at org.openqa.selenium.remote.RemoteWebElement.findElement(RemoteWebElement.java:185)
	at org.openqa.selenium.remote.RemoteWebElement.findElementByXPath(RemoteWebElement.java:261)
	at org.openqa.selenium.By$ByXPath.findElement(By.java:353)
	at org.openqa.selenium.remote.RemoteWebElement.findElement(RemoteWebElement.java:181)
	at sun.reflect.GeneratedMethodAccessor10.invoke(Unknown Source)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at org.openqa.selenium.support.pagefactory.internal.LocatingElementHandler.invoke(LocatingElementHandler.java:51)
	at com.sun.proxy.$Proxy44.findElement(Unknown Source)
	at org.xwiki.appwithinminutes.test.po.ApplicationClassEditPage.addField(ApplicationClassEditPage.java:104)
	at org.xwiki.test.ui.appwithinminutes.PageClassFieldTest.testSingleSelection(PageClassFieldTest.java:99)