Bug 331685 Unset context from model if widget creation failed since the context is disposed
If the renderer fails to create a widget for a given element, it will dispose its created context (if any) prior to returning from the call. If we try to deconstruct the element later, it will fail with an NPE because the context has been disposed and getParent() will return null.
tests/org.eclipse.e4.ui.tests/src/org/eclipse/e4/ui/tests/workbench/PartRenderingEngineTests.java
MPartStack partStack = BasicFactoryImpl.eINSTANCE.createPartStack();
		sashContainer.getChildren().add(partStack);
		sashContainer.setSelectedElement(partStack);
		MPart partA = BasicFactoryImpl.eINSTANCE.createPart();
		partStack.getChildren().add(partA);
		partStack.setSelectedElement(partA);
		MPart partB = BasicFactoryImpl.eINSTANCE.createPart();
		partStack.getChildren().add(partB);
		application.setContext(appContext);
		appContext.set(MApplication.class.getName(), application);
		wb = new E4Workbench(application, appContext);
		wb.createAndRunUI(window);
		assertNotNull(partA.getWidget());
		assertNull(partB.getWidget());
		partStack.setSelectedElement(partB);
		assertNotNull(partA.getWidget());
		assertNotNull(partB.getWidget());
		application.getContext().get(IPresentationEngine.class)
				.removeGui(sashContainer);
		assertEquals(partB, partStack.getSelectedElement());
	}
	public void testBug327701() {
		MApplication application = ApplicationFactoryImpl.eINSTANCE
				.createApplication();
		MWindow window = BasicFactoryImpl.eINSTANCE.createWindow();
		application.getChildren().add(window);
		application.setSelectedElement(window);
		MPartStack partStack = BasicFactoryImpl.eINSTANCE.createPartStack();
		window.getSharedElements().add(partStack);
		MPart part1 = BasicFactoryImpl.eINSTANCE.createPart();