Add null check to getInstanceID.

tools.descartes.prisma.core/src/tools/descartes/prisma/core/impl/CDOModelRepository.java

@@ -230,9 +230,12 @@ public class CDOModelRepository implements ModelRepository {
 
 	@Override
 	public String getInstanceID(Object object) {
-		StringBuilder out = new StringBuilder();
-		CDOIDUtil.write(out, ((CDOObject) object).cdoID());
-		return out.toString();
+		if (object != null) {
+			StringBuilder out = new StringBuilder();
+			CDOIDUtil.write(out, ((CDOObject) object).cdoID());
+			return out.toString();
+		}
+		return null;
 	}
 
 	@Override