NPE when opening/closing empty *.aadl file

[lwrage]

To reproduce just create a new empty file with extension aadl.
DefaultNodelChangeNotifier executes a getContents().get(0) on an empty resource.

!ENTRY org.eclipse.ui.workbench 4 2 2017-08-29 16:29:51.081
!MESSAGE Problems occurred when invoking code from plug-in: "org.eclipse.ui.workbench".
!STACK 0
org.eclipse.emf.common.util.BasicEList$BasicIndexOutOfBoundsException: index=0, size=0
	at org.eclipse.emf.common.util.BasicEList.get(BasicEList.java:346)
	at org.osate.ge.internal.services.impl.DefaultModelChangeNotifier$2.modelChanged(DefaultModelChangeNotifier.java:83)
	at org.osate.ge.internal.ui.xtext.OpenAadlResources$4.modelChanged(OpenAadlResources.java:229)
	at org.osate.ge.internal.ui.xtext.OpenAadlResources$2.exec(OpenAadlResources.java:148)
	at org.osate.ge.internal.ui.xtext.OpenAadlResources$2.exec(OpenAadlResources.java:1)
	at org.eclipse.xtext.resource.OutdatedStateManager.exec(OutdatedStateManager.java:91)
	at org.eclipse.xtext.ui.editor.model.XtextDocument$XtextDocumentLocker.internalReadOnly(XtextDocument.java:520)