Use synchronized double lock in DefaultMenuService

The root menu creation was not protected via a synchronized double lock,
which could have created problems if initializion was attempted by
multiple threads.

Author: hinerm

src/main/java/org/scijava/menu/DefaultMenuService.java

@@ -173,12 +173,19 @@ private synchronized void addModules(final Collection<ModuleInfo> items) {
 	 */
 	private HashMap<String, ShadowMenu> rootMenus() {
 		if (rootMenus == null) {
-			rootMenus = new HashMap<String, ShadowMenu>();
-
-			final List<ModuleInfo> allModules = moduleService.getModules();
-			addModules(allModules);
+			initRootMenus();
 		}
 		return rootMenus;
 	}
 
+	/** Initializes {@link #rootMenus}. */
+	private synchronized void initRootMenus() {
+		if (rootMenus != null) return;
+		final HashMap<String, ShadowMenu> map = new HashMap<String, ShadowMenu>();
+
+		final List<ModuleInfo> allModules = moduleService.getModules();
+		addModules(allModules);
+		rootMenus = map;
+	}
+
 }