I Get a "Vmdb error -18: Null cross reference" Error When Attempting to Remove a Farm

Case 1: One user creates a farm and leave the VirtualCenter client running. Another user then removes the created farm from another VirtualCenter client. For example:

1. Open two VirtualCenter clients and connect them to the same VirtualCenter server.
2. Create a new farm with one VirtualCenter client (VCA), and delete the new one from another VirtualCenter client (VCB).
3. VCA displays the error.

Case 2: Two users attempt to delete the same farm but one user clicks the confirmation button first. For example:

1. Open two VirtualCenter clients and connect them to the same VirtualCenter server.
2. Create a new farm with one VirtualCenter client (VCA).
3. Right-click the new farm in VCA and choose Remove. Leave the Confirm Remove Farm dialog box open. Do not click Yes yet.
4. In another client (VCB), right-click the newly-created farm and choose Remove. Click Yes to confirm the removal.
5. VCA gets the error Null cross reference.
   Note: If you connect the farms in the opposite order, you get the same result except that VCB displays the error.

Case 3: Two users try to remove the same farm at the same time. For example:

1. Open two VirtualCenter clients and connect them to the same VirtualCenter server.
2. Create a new farm with one VirtualCenter client (VCA).
3. Right-click -the new farm in VCA and choose Remove. Leave the Confirm Remove Farm dialog box open. Do not click Yes yet.
4. In another client (VCB), right-click the newly-created farm and choose Remove. Do not click Yes yet.
5. Now click Yes for both clients simultaneously.
6. The VirtualCenter client that commits the transaction first does not recieve the error, but another VirtualCenter client does receive the error.

To fix this problem, you need to upgrade to VirtualCenter version 1.3 or later.