WorkbookEditorsHandler: Fix NPE in case name is null

Regression was introduced in 03277d3,
where getTitle() was accidentally changed to getName().

Revert that back to getTitle().

getTitle() is already null-safe in most cases, but to be 100% safe,
guard it using Util.safeString() again.

Fixes #1275.

bundles/org.eclipse.ui.workbench/Eclipse UI/org/eclipse/ui/internal/WorkbookEditorsHandler.java

@@ -52,6 +52,7 @@
 import org.eclipse.ui.commands.ICommandService;
 import org.eclipse.ui.dialogs.SearchPattern;
 import org.eclipse.ui.dialogs.StyledStringHighlighter;
+import org.eclipse.ui.internal.util.Util;
 import org.eclipse.ui.themes.ITheme;
 
 /**
@@ -163,7 +164,7 @@ private List<EditorReference> getParts(WorkbenchPage page) {
 	private Map<EditorReference, String> generateColumnLabelTexts(List<EditorReference> editorReferences) {
 		Map<EditorReference, String> editorReferenceLabelTexts = new HashMap<>(editorReferences.size());
 		Map<String, List<EditorReference>> collisionsMap = editorReferences.stream()
-				.collect(Collectors.groupingBy(r -> r.getName()));
+				.collect(Collectors.groupingBy(r -> Util.safeString(r.getTitle())));
 
 		for (Entry<String, List<EditorReference>> groupedEditorReferences : collisionsMap.entrySet()) {
 			if (groupedEditorReferences.getValue().size() == 1) {