Probably because of these lines in build.gradle:

testCompile group: 'org.testfx', name: 'testfx-core', version: '4.0.+'
testCompile group: 'org.testfx', name: 'testfx-junit', version: '4.0.+'

A new version of testfx was released on March 23rd (4.0.6-alpha) that seems to have a bug that's causing this.