SubDiagram example not loading anymore

[alexanderpann]

One of the last diagram PRs probably broke it:

java.lang.NullPointerException: Cannot invoke "de.itemis.mps.editor.diagram.runtime.jgraph.MyGraph.getRootDiagramModel()" because the return value of "de.itemis.mps.editor.diagram.runtime.jgraph.DiagramCreationContext.getRootGraph()" is null
	at d.i.m.e.diagram.demolang//de.itemis.mps.editor.diagram.demolang.editor.SubComponent_EditorBuilder_a.lambda$createDiagramNode_0$0(SubComponent_EditorBuilder_a.java:102)
	at d.i.m.e.diagram.runtime//de.itemis.mps.editor.diagram.runtime.ContextVariables.lambda$withValue$0(ContextVariables.java:39)
	at d.i.m.e.diagram.runtime//de.itemis.mps.editor.diagram.runtime.ContextVariables.enterNewContext(ContextVariables.java:30)
	at d.i.m.e.diagram.runtime//de.itemis.mps.editor.diagram.runtime.ContextVariables.withValue(ContextVariables.java:37)
	at d.i.m.e.diagram.demolang//de.itemis.mps.editor.diagram.demolang.editor.SubComponent_EditorBuilder_a.createDiagramNode_0(SubComponent_EditorBuilder_a.java:62)
	at d.i.m.e.diagram.demolang//de.itemis.mps.editor.diagram.demolang.editor.SubComponent_EditorBuilder_a.createDiagramNode_1(SubComponent_EditorBuilder_a.java:109)
	at d.i.m.e.diagram.demolang//de.itemis.mps.editor.diagram.demolang.editor.SubComponent_EditorBuilder_a.createCell(SubComponent_EditorBuilder_a.java:55)
	at d.i.m.e.diagram.demolang//de.itemis.mps.editor.diagram.demolang.editor.SubComponent_Editor.createEditorCell(SubComponent_Editor.java:12)
	at jetbrains.mps.nodeEditor.cells.EditorCellFactoryImpl.createCell(EditorCellFactoryImpl.java:136)
	at jetbrains.mps.nodeEditor.cells.EditorCellFactoryImpl.createEditorCell_internal(EditorCellFactoryImpl.java:102)
	at jetbrains.mps.nodeEditor.cells.EditorCellFactoryImpl.createEditorCell(EditorCellFactoryImpl.java:89)
	at jetbrains.mps.nodeEditor.EditorManager.createEditorCell_internal(EditorManager.java:370)
	at jetbrains.mps.nodeEditor.EditorManager.createEditorCell(EditorManager.java:260)
	at jetbrains.mps.nodeEditor.EditorManager.createRootCell(EditorManager.java:114)
	at jetbrains.mps.nodeEditor.updater.UpdateSessionImpl.performUpdate(UpdateSessionImpl.java:157)
	at jetbrains.mps.nodeEditor.updater.UpdaterImpl.lambda$updateRootCell$0(UpdaterImpl.java:133)
	at