"Manage testcases" closes after deleting a testcase so if you have some testcases which isn't assigned to a requirement you have to open manage testcases for each testcase you wish to delete.
1) Add 2 testcases
2) Open manage testcases
3) Delete a testcase
You should now see that the manage testcases dialog is closed and has to be reopened in order to delete the last testcase.