error handling improved - empty image id is a valid use case
diff --git a/plugins/org.eclipse.graphiti.ui/src/org/eclipse/graphiti/ui/internal/services/impl/ImageService.java b/plugins/org.eclipse.graphiti.ui/src/org/eclipse/graphiti/ui/internal/services/impl/ImageService.java
index bbf267f..3ff6ad8 100644
--- a/plugins/org.eclipse.graphiti.ui/src/org/eclipse/graphiti/ui/internal/services/impl/ImageService.java
+++ b/plugins/org.eclipse.graphiti.ui/src/org/eclipse/graphiti/ui/internal/services/impl/ImageService.java
@@ -51,7 +51,7 @@
 	public Image getImageForId(String imageId) {
 
 		if (imageId == null)
-			throw new IllegalArgumentException("imageId must not be null");
+			return null;
 
 		// if image already available take it
 		ImageRegistry imageRegistry = GraphitiUIPlugin.getDefault().getImageRegistry();