If you have a child.fxml which has a controller which gets included into a parent.fxml which does not have one you get an NPE like this:
Child controller created
java.lang.NullPointerException
/Users/tomschindl/Documents/e4_workspaces/efxclipse/testloader/bin/test/Parent.fxml:14
at javafx.fxml.FXMLLoader.getControllerFields(FXMLLoader.java:2538)
at javafx.fxml.FXMLLoader.access$1400(FXMLLoader.java:68)
at javafx.fxml.FXMLLoader$IncludeElement.constructValue(FXMLLoader.java:946)
at javafx.fxml.FXMLLoader$ValueElement.processStartElement(FXMLLoader.java:567)
at javafx.fxml.FXMLLoader.processStartElement(FXMLLoader.java:2313)
at javafx.fxml.FXMLLoader.load(FXMLLoader.java:2130)
at javafx.fxml.FXMLLoader.load(FXMLLoader.java:2027)
at javafx.fxml.FXMLLoader.load(FXMLLoader.java:2741)
at javafx.fxml.FXMLLoader.load(FXMLLoader.java:2720)
at javafx.fxml.FXMLLoader.load(FXMLLoader.java:2706)
at javafx.fxml.FXMLLoader.load(FXMLLoader.java:2693)
at javafx.fxml.FXMLLoader.load(FXMLLoader.java:2682)
at test.TestSubcontroller.start(TestSubcontroller.java:16)
at com.sun.javafx.application.LauncherImpl$5.run(LauncherImpl.java:319)
at com.sun.javafx.application.PlatformImpl$5.run(PlatformImpl.java:206)
at com.sun.javafx.application.PlatformImpl$4.run(PlatformImpl.java:173)
at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:76)
java.lang.NullPointerException
at javafx.fxml.FXMLLoader.getControllerFields(FXMLLoader.java:2538)
at javafx.fxml.FXMLLoader.access$1400(FXMLLoader.java:68)
at javafx.fxml.FXMLLoader$IncludeElement.constructValue(FXMLLoader.java:946)
at javafx.fxml.FXMLLoader$ValueElement.processStartElement(FXMLLoader.java:567)
at javafx.fxml.FXMLLoader.processStartElement(FXMLLoader.java:2313)
at javafx.fxml.FXMLLoader.load(FXMLLoader.java:2130)
at javafx.fxml.FXMLLoader.load(FXMLLoader.java:2027)
at javafx.fxml.FXMLLoader.load(FXMLLoader.java:2741)
at javafx.fxml.FXMLLoader.load(FXMLLoader.java:2720)
at javafx.fxml.FXMLLoader.load(FXMLLoader.java:2706)
at javafx.fxml.FXMLLoader.load(FXMLLoader.java:2693)
at javafx.fxml.FXMLLoader.load(FXMLLoader.java:2682)
at test.TestSubcontroller.start(TestSubcontroller.java:16)
at com.sun.javafx.application.LauncherImpl$5.run(LauncherImpl.java:319)
at com.sun.javafx.application.PlatformImpl$5.run(PlatformImpl.java:206)
at com.sun.javafx.application.PlatformImpl$4.run(PlatformImpl.java:173)
at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:76)
Child controller created
java.lang.NullPointerException
/Users/tomschindl/Documents/e4_workspaces/efxclipse/testloader/bin/test/Parent.fxml:14
at javafx.fxml.FXMLLoader.getControllerFields(FXMLLoader.java:2538)
at javafx.fxml.FXMLLoader.access$1400(FXMLLoader.java:68)
at javafx.fxml.FXMLLoader$IncludeElement.constructValue(FXMLLoader.java:946)
at javafx.fxml.FXMLLoader$ValueElement.processStartElement(FXMLLoader.java:567)
at javafx.fxml.FXMLLoader.processStartElement(FXMLLoader.java:2313)
at javafx.fxml.FXMLLoader.load(FXMLLoader.java:2130)
at javafx.fxml.FXMLLoader.load(FXMLLoader.java:2027)
at javafx.fxml.FXMLLoader.load(FXMLLoader.java:2741)
at javafx.fxml.FXMLLoader.load(FXMLLoader.java:2720)
at javafx.fxml.FXMLLoader.load(FXMLLoader.java:2706)
at javafx.fxml.FXMLLoader.load(FXMLLoader.java:2693)
at javafx.fxml.FXMLLoader.load(FXMLLoader.java:2682)
at test.TestSubcontroller.start(TestSubcontroller.java:16)
at com.sun.javafx.application.LauncherImpl$5.run(LauncherImpl.java:319)
at com.sun.javafx.application.PlatformImpl$5.run(PlatformImpl.java:206)
at com.sun.javafx.application.PlatformImpl$4.run(PlatformImpl.java:173)
at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:76)
java.lang.NullPointerException
at javafx.fxml.FXMLLoader.getControllerFields(FXMLLoader.java:2538)
at javafx.fxml.FXMLLoader.access$1400(FXMLLoader.java:68)
at javafx.fxml.FXMLLoader$IncludeElement.constructValue(FXMLLoader.java:946)
at javafx.fxml.FXMLLoader$ValueElement.processStartElement(FXMLLoader.java:567)
at javafx.fxml.FXMLLoader.processStartElement(FXMLLoader.java:2313)
at javafx.fxml.FXMLLoader.load(FXMLLoader.java:2130)
at javafx.fxml.FXMLLoader.load(FXMLLoader.java:2027)
at javafx.fxml.FXMLLoader.load(FXMLLoader.java:2741)
at javafx.fxml.FXMLLoader.load(FXMLLoader.java:2720)
at javafx.fxml.FXMLLoader.load(FXMLLoader.java:2706)
at javafx.fxml.FXMLLoader.load(FXMLLoader.java:2693)
at javafx.fxml.FXMLLoader.load(FXMLLoader.java:2682)
at test.TestSubcontroller.start(TestSubcontroller.java:16)
at com.sun.javafx.application.LauncherImpl$5.run(LauncherImpl.java:319)
at com.sun.javafx.application.PlatformImpl$5.run(PlatformImpl.java:206)
at com.sun.javafx.application.PlatformImpl$4.run(PlatformImpl.java:173)
at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:76)