Fix layout definitions export

2018-01-04 15:19:14,162 ERROR [http-nio-0.0.0.0-8080-exec-1] [freemarker.runtime] Error executing FreeMarker template
FreeMarker template error:
The following has evaluated to null or missing:
==> rowDef.getWidgets  [in template "jar:file:/opt/nuxeo-server-tomcat-10.1-SNAPSHOT/nxserver/bundles/nuxeo-platform-forms-layout-export-10.1-SNAPSHOT.jar!/org/nuxeo/ecm/platfo
rm/forms/layout/export/layouts.ftl" at line 117, column 25]

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Tip: It's the step after the last dot that caused this error, not those before it.
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Tip: If the failing expression is known to be legally refer to something that's sometimes null or missing, either specify a default value like myOptionalVar!myDefault, or use <
#if myOptionalVar??>when-present<#else>when-missing</#if>. (These only cover the last step of the expression; to cover the whole expression, use parenthesis: (myOptionalVar.foo
)!myDefault, (myOptionalVar.foo)??
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FTL stack trace ("~" means nesting-related):
        - Failed at: #list rowDef.getWidgets() as widgetName  [in template "jar:file:/opt/nuxeo-server-tomcat-10.1-SNAPSHOT/nxserver/bundles/nuxeo-platform-forms-layout-export-
10.1-SNAPSHOT.jar!/org/nuxeo/ecm/platform/forms/layout/export/layouts.ftl" at line 117, column 18]