Fix NPE with additional contributions without icon

Change-Id: I21e9028412f888f069badb75ba2de5a50b94b5fd
Signed-off-by: Markus Duft <markus.duft@ssi-schaefer.com>
diff --git a/org.eclipse.tea.core/src/org/eclipse/tea/core/services/TaskingAdditionalMenuEntryProvider.java b/org.eclipse.tea.core/src/org/eclipse/tea/core/services/TaskingAdditionalMenuEntryProvider.java
index 4554682..16fb6c1 100644
--- a/org.eclipse.tea.core/src/org/eclipse/tea/core/services/TaskingAdditionalMenuEntryProvider.java
+++ b/org.eclipse.tea.core/src/org/eclipse/tea/core/services/TaskingAdditionalMenuEntryProvider.java
@@ -49,12 +49,18 @@
 		}
 
 		protected static String getIconBundle(String iconUri) {
+			if (iconUri == null) {
+				return null;
+			}
 			Assert.isLegal(iconUri.startsWith("platform:/plugin"), "URI must be of platform:/plugin scheme");
 			String sub = iconUri.substring("platform:/plugin/".length());
 			return sub.substring(0, sub.indexOf('/'));
 		}
 
 		protected static String getIconPath(String iconUri) {
+			if (iconUri == null) {
+				return null;
+			}
 			Assert.isLegal(iconUri.startsWith("platform:/plugin"), "URI must be of platform:/plugin scheme");
 			String sub = iconUri.substring("platform:/plugin/".length());
 			return sub.substring(sub.indexOf('/') + 1);