DefaultUIService: fix bug in default UI logic

The discoverUIs() initialization logic checks the scijava.ui property,
and assigns the default UI accordingly. But the getDefaultUI() method
was not forcing initialization before checking the value of the
defaultUI field, so it was null even when the property was set.

This commit forces initialization before getDefaultUI() proceeds,
so that the correct default UI is returned.

Author: ctrueden

src/main/java/org/scijava/ui/DefaultUIService.java

@@ -190,6 +190,7 @@ public boolean isHeadless() {
 
 	@Override
 	public UserInterface getDefaultUI() {
+		if (!initialized) discoverUIs();
 		if (isHeadless()) return uiMap().get(HeadlessUI.NAME);
 		if (defaultUI != null) return defaultUI;
 		return uiList().isEmpty() ? null : uiList().get(0);