While this is not an IntelliJ support forum I may be able to help you if you share the FXML content.
FWIW I have had many issues with the embedded FXML editor in all versions of IDEA up to and including the latest RC. The IDE does provide an alternative of context-clicking (ie right click) an FXML file and selecting Open in Scenebuilder.
The potential complication with this workaround is that your dev class-path will not be available to SceneBuilder for resolving custom component classes imported in your FXML. IF you have that specific issue when trying the above you will find help on SO or you can ask again here.