Fix NullPointerException when exporting layout definition in json

http://localhost:8080/nuxeo/site/layout-manager/layouts/json?layoutName=thumbnail_listing

11:50:25,455 ERROR [DefaultNuxeoExceptionHandler] Cannot forward to error page: response is already committed
12:04:17,917 WARN  [WebEngineExceptionMapper] Exception in JAX-RS processing
java.lang.NullPointerException
	at org.nuxeo.ecm.platform.forms.layout.io.JSONLayoutExporter.exportToJson(JSONLayoutExporter.java:1091)
	at org.nuxeo.ecm.platform.forms.layout.io.JSONLayoutExporter.exportToJson(JSONLayoutExporter.java:954)
	at org.nuxeo.ecm.platform.forms.layout.io.JSONLayoutExporter.exportToJson(JSONLayoutExporter.java:746)
	at org.nuxeo.ecm.platform.forms.layout.export.LayoutResource.getAsJson(LayoutResource.java:108)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:606)
	at com.sun.jersey.spi.container.JavaMethodInvokerFactory$1.invoke(JavaMethodInvokerFactory.java:60)