Steps to reproduce

• A] run a 6.3 build on jenkins https://jenkins.magnolia-cms.com/job/platform/job/ui/
• B] run RowDragger test locally in IDEA in a loop until failure

   

Expected results

Test/build always passes.

Actual resultshttps://jenkins.magnolia-cms.com/blue/organizations/jenkins/platform%2Fui/detail/PR-2350/21/pipeline#step-421-log-1152

[ERROR] RowDraggerTest.showWarningMessageWhenDropConstraintIsNotAllowedAt:129 » NullPointer Cannot invoke "com.vaadin.server.AbstractClientConnector.addExtension(com.vaadin.server.Extension)" because "target" is null

Workaround

Re-trigger the build until the build is successful.

Development notes

The failures are more often after update of a junit runner. There don't seem to be any previous tests which would not clean a resources. It might be connected to reusing threads etc.

As the investigation of the cause was not successful we suggest to avoid static Vaadin calls to show notifications which would make testing easier.

We replace usages of Notification.show only in classes with no public ctors. The rest can be tackled when the constructors need to be changed.