ShadowMenu: avoid NPE when child module is null

See: http://fiji.sc/bugzilla/show_bug.cgi?id=944

Of course, it is quite odd for the child module to be null. It may be a
symptom of a larger issue. But it's still better not to throw NPE here.

Author: ctrueden

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

@@ -529,14 +529,15 @@ private ShadowMenu addChild(final ModuleInfo info, final int depth) {
 		if (!leaf) child.addChild(info, depth + 1);
 		else if (existingChild != null) {
 			if (log != null) {
-				if (info.getPriority() == existingChild.getModuleInfo().getPriority()) {
+				final ModuleInfo childInfo = existingChild.getModuleInfo();
+				if (childInfo != null && info.getPriority() == childInfo.getPriority())
+				{
 					log.warn("ShadowMenu: menu item already exists:\n\texisting: " +
-						existingChild.getModuleInfo() + "\n\t ignored: " + info);
+						childInfo + "\n\t ignored: " + info);
 				}
 				else {
 					log.debug("ShadowMenu: higher-priority menu item already exists:\n" +
-						"\texisting: " + existingChild.getModuleInfo() + "\n\t ignored: " +
-						info);
+						"\texisting: " + childInfo + "\n\t ignored: " + info);
 				}
 			}
 		}