Fixed potential NPE in traceability matrix

Depending on the condition of the workbench, the traceability matrix
could throw NPEs when either the trace model was not yet loaded or the
table was not initialised. This commit fixes both issues.

bundles/org.eclipse.capra.ui.matrix/src/org/eclipse/capra/ui/matrix/views/TraceabilityMatrixView.java

diff --git a/bundles/org.eclipse.capra.ui.matrix/src/org/eclipse/capra/ui/matrix/views/TraceabilityMatrixView.java b/bundles/org.eclipse.capra.ui.matrix/src/org/eclipse/capra/ui/matrix/views/TraceabilityMatrixView.java
index 2270784..d07e7c8 100644
--- a/bundles/org.eclipse.capra.ui.matrix/src/org/eclipse/capra/ui/matrix/views/TraceabilityMatrixView.java
+++ b/bundles/org.eclipse.capra.ui.matrix/src/org/eclipse/capra/ui/matrix/views/TraceabilityMatrixView.java
@@ -125,12 +125,12 @@
 	private Composite parent;
 
 	private ResourceSet resourceSet = new ResourceSetImpl();
-	private EObject traceModel = null;
-	private EObject artifactModel = null;
 
 	private final TraceMetaModelAdapter traceAdapter = ExtensionPointHelper.getTraceMetamodelAdapter().get();
 	private TraceMetaModelAdapter metamodelAdapter = ExtensionPointHelper.getTraceMetamodelAdapter().get();
 	private TracePersistenceAdapter persistenceAdapter = ExtensionPointHelper.getTracePersistenceAdapter().get();
+	private EObject traceModel = null;
+	private EObject artifactModel = null;
 	private ArtifactHelper artifactHelper;
 
 	private TraceabilityMatrixDataProvider bodyDataProvider;
@@ -148,7 +148,6 @@
 		public void selectionChanged(IWorkbenchPart part, ISelection selection) {
 			populateSelectedModels(part, selection);
 			if (traceMatrixTable != null && selectionModified) {
-				Composite parent = traceMatrixTable.getParent();
 				updateTraceabilityMatrix();
 			}
 		}
@@ -254,7 +253,9 @@
 
 	@Override
 	public void setFocus() {
-		traceMatrixTable.setFocus();
+		if (traceMatrixTable != null) {
+			traceMatrixTable.setFocus();
+		}
 	}
 
 	/**
@@ -270,6 +271,7 @@
 
 		this.artifactModel = persistenceAdapter.getArtifactWrappers(resourceSet);
 		this.artifactHelper = new ArtifactHelper(this.artifactModel);
+		this.traceModel = persistenceAdapter.getTraceModel(resourceSet);
 
 		if (!selectedModels.isEmpty()) {
 			// Show the matrix for the selected objects
@@ -292,7 +294,9 @@
 			}
 		} else {
 			// Without a selection, show a matrix of all traces
-			traces = metamodelAdapter.getAllTraceLinks(traceModel);
+			if (traceModel != null) {
+				traces = metamodelAdapter.getAllTraceLinks(traceModel);
+			}
 		}
 		// If the current selection does not contain elements from the trace model, it
 		// is possible that the list of traces is empty. By catching this case, we only