Error Markers: Avoid potential NPE

* Can be caused by dangling objects that e.g. stem from partical
  cross-project copies

Issue-Ref: 2845
Issue-Url: https://af3-developer.fortiss.org/issues/2845

Signed-off-by: Simon Barner <barner@fortiss.org>

org.fortiss.tooling.kernel.ui/src/org/fortiss/tooling/kernel/ui/internal/views/.ratings

@@ -5,7 +5,7 @@ LibraryView.java 6f7273ee49b9049c7267c129bd59ed27ec5872e2 GREEN
 LibraryViewDragSourceAdapter.java 56ef61b214ef5d6cb5b751791a92158bda0391ec GREEN
 LinkWithEditorPartListener.java c5ab74424378e7b158a805c4dd14fc03c8abeded GREEN
 MarkerViewContentProvider.java 297d9a5c355047796dc938eb909fd1cf27a2cbf3 GREEN
-MarkerViewPart.java 074e8e65c46daf50749d26d5eb866eef3eac2f2d GREEN
+MarkerViewPart.java cbb650271b6877af205421b7cb11f930440a7ef9 YELLOW
 NavigatorNewMenu.java a35e391960d1dacbe7f77982e53e1891e9382d5a GREEN
 NavigatorTreeContentProvider.java 1bdefa28c5e2c2619cabbcd1946269a52db34e6c GREEN
 NavigatorTreeContentSorter.java 76fc94e441ac1a8f2c5ae573f0e2b1e6fa73c31a GREEN

org.fortiss.tooling.kernel.ui/src/org/fortiss/tooling/kernel/ui/internal/views/MarkerViewPart.java

@@ -38,6 +38,7 @@ import org.eclipse.swt.widgets.Composite;
 import org.eclipse.ui.part.ViewPart;
 import org.fortiss.tooling.kernel.extension.data.IConstraintViolation;
 import org.fortiss.tooling.kernel.extension.data.IConstraintViolation.ESeverity;
+import org.fortiss.tooling.kernel.extension.data.ITopLevelElement;
 import org.fortiss.tooling.kernel.service.IPersistencyService;
 import org.fortiss.tooling.kernel.ui.extension.IModelElementHandler;
 import org.fortiss.tooling.kernel.ui.internal.MarkerService;
@@ -95,9 +96,9 @@ public class MarkerViewPart extends ViewPart {
 				if(cell.getElement() instanceof IConstraintViolation) {
 					IConstraintViolation<? extends EObject> violation =
 							(IConstraintViolation<? extends EObject>)cell.getElement();
+					IModelElementHandlerService hs = IModelElementHandlerService.getInstance();
 					IModelElementHandler<EObject> handler =
-							IModelElementHandlerService.getInstance().getModelElementHandler(
-									violation.getSource());
+							hs.getModelElementHandler(violation.getSource());
 					cell.setText(handler != null ? handler.getName(violation.getSource()) : "");
 				} else {
 					cell.setText("");
@@ -123,13 +124,17 @@ public class MarkerViewPart extends ViewPart {
 				if(cell.getElement() instanceof IConstraintViolation) {
 					IConstraintViolation<? extends EObject> violation =
 							(IConstraintViolation<? extends EObject>)cell.getElement();
-					EObject rootElement =
-							IPersistencyService.getInstance()
-									.getTopLevelElementFor(violation.getSource())
-									.getRootModelElement();
-					IModelElementHandler<EObject> handler =
-							IModelElementHandlerService.getInstance().getModelElementHandler(
-									rootElement);
+					IPersistencyService ps = IPersistencyService.getInstance();
+					ITopLevelElement topElement = ps.getTopLevelElementFor(violation.getSource());
+
+					IModelElementHandler<EObject> handler = null;
+					EObject rootElement = null;
+					if(topElement != null) {
+						rootElement = topElement.getRootModelElement();
+
+						IModelElementHandlerService hs = IModelElementHandlerService.getInstance();
+						handler = hs.getModelElementHandler(rootElement);
+					}
 					cell.setText(handler != null ? handler.getName(rootElement) : "");
 				} else {
 					cell.setText("");