Anpassen der geg. Lösung an den Null-Testfall

SimpleActivityStack/src/solution/SimpleActivityStack.java

@@ -6,6 +6,9 @@ public class SimpleActivityStack extends SimpleStack<SimpleActivity> {
 
 	@Override
 	public int push(SimpleActivity activity) {
+		if (activity == null) {
+			return 0;
+		}
 		if (size() > 0) {
 			super.get(size() - 1).passivate();
 		}