I had sometimes the same Exception.
When i create a FXML-File with the Wizard in Eclipse, i write example.fxml in the name field. Eclipse creates a file, like example.fxml.fxml. With this mistake, the FXMLLoader can't find the right FXML-File. So, my tip, check the name of the FXML-Filename in your start-Method and the real Name of File.
Hopes i could help. Good luck.