[Minor] Don't allow to call Activity.ensureModifiable() no a sub activity

model/src/main/java/li/strolch/model/activity/Activity.java

@@ -740,6 +740,8 @@ public class Activity extends AbstractStrolchRootElement
 
 	@Override
 	public Activity ensureModifiable() {
+		if (!this.isRootElement())
+			throw new IllegalStateException("Only call this method on the root element!");
 		if (isReadOnly())
 			return getClone(true);
 		return this;