build number:2.6.0.v20100507-0630

steps to reproduce:
1. insert a table, bind with dataset
2. add a group without key expression, click ok and save

expected result:
error icon in outline and error in problem view

actual result:
no error.
fix the valuerequiredvalidator. it can process expression with null value now.
2.6.0.v20100520-0630