Steps to reproduce:
1. On Windows, create a fxml file and save it with non-ascii file name.
2. make .fxml associate with SceneBuilder if SceneBuilder is not the default.
3. double click on the fxml file.

The warning dialog pops up, says the the file doesn't exist.
If rename the file to ascii, SceneBuilder is able to open it.
And if launch SceneBuilder first, and them open the file through menu "File->Open..", it works fine.